Let's assume I narrow it to 500mm, length will be 550mm, height 270mm including wheel clearance.

In theory I could reduce the width further but it will limit my weapon options and I want stuff to end upon top.
The height is pretty fixed, and the length of ramp is constrained by the big motors and the necessary angle to cover them.

I have some nice Alligrator green 12.7mm HDPE but the floor can be aluminium. Will have to do some sums.

This is exactly why I wanted to build a full size ply version to see how it all comes together.