How to Fix the “Too Thick Wall” Problem When Installing an Accordion Boot

If you need to cut an opening for your box truck pass-through and want to use the accordion boot. But you find the wall’s too thick, there’s no proper flange, and the boot’s spring clip has nothing to grab. Don’t worry here is a solution that works.

The Problem You’re Dealing With

Accordion boots are made to clamp onto a flange — a thin, uniform edge the spring steel clip can bite into. If your wall is too thick or doesn’t have that edge, the boot won’t attach properly. In its native state the boot has nothing to grab.

The Fix: Make Your Own Flange with FRP Panel

Instead of trying to modify your truck wall, you can create a flange surface the boot can grip using FRP (Fiberglass Reinforced Panel). FRP is strong, waterproof, lightweight, and easy to work with if you know how to cut it right. It is also easily available at most hardware stores. It normally come in 4′ x 8′ sheets.

For this project FRP is used but this is not the only material that will work with the project. Here are some alternative to also consider:

• Sheet metal (16-18 Guage)
• Steel flat bar – May require welding
• Aluminum composite panel (ACP/Dibond® style)
• HDPE “marine board” (StarBoard®), 1/8″
• Aluminum flat bar – Can have issue with radius corners

Here’s What You’ll Need

Materials:

• FRP panel, cut larger than your passthrough opening
• Heavy-duty adhesive (FRP/metal compatible)
• Stainless steel screws
• Accordion boot with spring clip
• Polyurethane sealant

Tools:

• Circular saw with carbide-tipped blade (straight cuts)
• Jigsaw with fine-toothed blade (interior cutout)
• Hole saw
• Drill/driver
• Measuring tape
• Marker
• Clamps
• Safety gear (goggles, gloves, dust mask)

Step-by-Step: My Recommended Method

1. Measure and Add Overlap

Measure your passthrough opening and add 2-3 inches all the way around. That’s the size your FRP panel needs to be.

2. Cut the FRP Panel

Use the right approach so you get clean, chip-free edges:

When laying out the opening keep in mind that you will need at minimum a 1″ radius in the corners. Of course larger is better. Use this as a guideline.

• Circular saw with a carbide-tipped blade for the outer cut.
• Jigsaw with fine teeth for the inner cut where the boot will attach.
• Use a hole saw to define the corners
• Clamp the panel to keep it steady.
• Cut with the smooth back side facing up to minimize chipping.
• Sand the edges smooth and seal them with clear urethane.

3. Bond the Panel to the Wall

Apply heavy-duty adhesive around the opening, press the FRP panel into place, then add stainless screws at even intervals for a secure mechanical hold.

4. Seal the Edges

Run a bead of polyurethane sealant around the outside of the FRP panel. This keeps water and dust out.

5. Attach the Accordion Boot

Now the fun part — press the boot’s spring clip over your new FRP flange. You’ll feel it snap in and hold firmly.

Why This Works

• You’re creating a perfect flange where none existed.
• FRP is thin enough for the boot to grip but strong enough to last.
• The seal is tight, weatherproof, and built to handle movement.

Bottom line: If your wall is too thick or doesn’t have a flange, don’t fight it — just make one. FRP gives you a professional finish and a boot connection you can count on.

References

1. Stabilit America. How to Cut FRP Panels. Jul 26, 2024. Available at: stabilitamerica.com. Practical cutting methods (circular saw with carbide blade for straights; fine-tooth jigsaw for internal/curved cuts). Accessed Aug 14, 2025.
2. Crane Composites. FRP Installation Guide (Walls/Ceilings/Laminated) [PDF 6876]. Includes recommended tools (fine-tooth carbide circular saw, jigsaw) and adhesive options. Available at: cranecomposites.com. Accessed Aug 14, 2025.
3. NIOSH/CDC. Fibrous Glass Dust – NIOSH Pocket Guide. Exposure limits and control guidance for fiberglass dust. Available at: cdc.gov. Accessed Aug 14, 2025.
4. OSHA. Synthetic Mineral Fibers – Overview. General safety resources for fiberglass work (PPE and exposure evaluation). Available at: osha.gov. Accessed Aug 14, 2025.
5. YouTube (Nomad Brad). BEST Cab Access Door | Box Truck Conversion. Demonstrates a cab-to-box passthrough build; useful context for FRP-flange workaround. Available at: youtube.com. Accessed Aug 14, 2025.

Disclaimer & Safety Notice

The information in this article is provided for educational purposes only. Vehicle modifications involve risk. Proceed at your own discretion and assume all responsibility for results, including any damage, injury, or loss that may occur.

No Professional Advice: This content is not engineering, mechanical, or legal advice. If you are unsure about any step, consult a qualified fabricator, body technician, or RV upfitter.

Warranty & Returns: Modifying products or using them outside their standard installation (including trimming, altering embedded clips, bonding to non-flange surfaces, or fabricating custom flanges) may void any manufacturer or store warranty and void return eligibility.

Fitment & Compatibility: Verify measurements, flange thickness, and material compatibility (adhesives/sealants with EPDM, FRP, aluminum, etc.) before cutting or bonding. Perform a test fit on scrap material to confirm the boot’s spring clip engages securely.

Structural & Safety Systems: Cutting body panels can affect structure and safety systems. Confirm that your opening and fastener layout do not interfere with airbags, seatbelts, wiring harnesses, VIN/emissions labels, or load-bearing members.

Adhesives & Sealants: Follow manufacturer instructions, surface prep, and cure times. Use appropriate PPE and ventilation. Check for chemical compatibility and temperature ratings for your climate.

Weatherproofing: Water intrusion can cause corrosion, mold, and electrical issues. Inspect fasteners and seals periodically and re-seal as needed, especially after extreme temperatures, snow/ice, or off-road use.

Tools & PPE: Cutting FRP/metals creates dust and sharp edges. Use eye/hand/respiratory protection, clamp workpieces, and follow safe tool practices.

Regulatory Compliance: Check local/state regulations, insurance requirements, and vehicle inspection rules before performing modifications that alter the cab/cargo barrier or body panels.

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