Making your own canopy would be an interesting exercise and if you're good with fibreglass then it could be a lot of fun.
The only advice I'd give: make sure the upright sections can take some weight, use some tubular steel in there if you can (and across the roof) and put LED strip lighting all around. A lip-over the tub (like the SnugTop, look in my garage) helps keep the rain from seeping into the gap between. Oh, an extra brake light built into the top of the rear would not be a bad idea.
Use lift-up windows all around if access is an issue, or sliding with security mesh for carrying animals.
As for drawers, there are a few blokes here that have made them, but the important thing to consider is the weight. One of our forum members had drawers in the vehicle that were so strong and heavy that unladen his car exceeded its gvm.
Good luck - keep us posted!