Mating can to box.
 

I have a 2000 26' u-haul that I'm going to convert into a motorhome home. What is everyone using to mount the cab to the box. A seal or gasket? Or heard mounting them together. I'm wanting to build a toy hauler with a pass through

You wanting to cut a hole through from the cab to the box? I think there are places you can buy the accordian boot gasket to do that. Probably google would help find those?

I used a Uni Grip product, One of these, Accordion Seals | Uni-Grip Inc., has room for movement between the box and the cab, and was pretty easy to work with.

Semper Gumby.

Frames flex. They twist and swivel.

For mating, please consider using a flexible coupling. An accordion affair. Something with plenty of give.

Let us know about your experiences.