Join cab to box for pass-thru on u-haul truck

[Ozrockrat]

Looks like you have your answer already but measure the gap and buy the boot rubber to suit. My truck has about 11” between the cab and box with a 2” lip on the cab and a 6” lip on the box. So I used the biggest boot rubber they listed (4” - 10”). Make sure you leave plenty of overlap to join them together.

When I was chasing it the manufacturer would not sell in reasonable size quantities hence the fix my Ambulance as a supplier.

 

[Mediatunes]

To fill the gap in the pass through the easiest solution is to use the accordion boot. It is designed to press on a thin wall or flange. Ideally the thickness should be +/- 1/8". Fiberglass and sheet metal are good materials. One thing to consider is to make sure both openings are exactly the same size. If they are different you will have a difficult time with installation. Another best practice is to radius the corners. The corners can be round or 45deg angle. Check out https://www.accordionboot.com for lots more information