Here's a link for some Fiberglass Caps, Inside has more than enough room to stand up (if you are under 6' tall)
http://www.fleetwest.net/ds365.html
I had a service truck in British Columbia for a couple of years with a "spacecap" brand cap on the back...
It was a Dodge 3500 SRW 4x4 QuadCab loaded to the gills with tools and junk that go with being a heavy equipment mechanic.
And the back door was a double clamshell type, that allowed for easy access to the work area
It would have made a great camper if it wasn't for all the greasy, smelly tools in the back... it certainly spent a lot of it's "free" time in 4wd...:smiley_drive: