I have custom built these using The Corps humvee

Step 1, I removed all add ons to the truck, guns, lights, brush guards, Etc. 

Step 2, I removed the body from the chassis (it's held together with philips head screws) the dash and windshield were also removed. 

Step 3, At this point the xacto knife is needed for carefully removing the rear doors, once the doors are removed, then the B post between the doors were removed, and relocated to the center of the opening of both doors, using plastic model glue, then hot melt glue on the back side for reinforcement. 

Step 4, with dremal tool in hand I removed all excess extrusions, bumps Etc. from the roof, I also removed the rear wall of the truck cab, filing is $kind=archive;required to smooth the cuts. 

Step 5, Next using 1/16" sheet plastic I made a solid smooth roof. The roof was pre measured and bends were made in the plastic by using my wife's electric oven. If you turn the oven on high and hold sheet plastic 1/2" above heating element in the area and direction you want to bend the plastic. It only takes a few seconds for the plastic to warm up enough to bend. Once the plastic is bent hold in place till cool. Be very careful this can cause serious injury. Check fit of the new roof and modify as needed. once finished set aside. 

Step 6, Hatch back; the top and back as on piece was measured and cut from 1/8" sheet plastic. The above bending process was done to get the big radius from the hatch top to hatch rear. Next two sides were measured using the top and back as a template as well as the rear truck body. the sides were cut then glued using model car glue. The hatch was put aside overnight for drying. 

Step 7, The roof and hatch are now ready to be glued on to truck body. the hatch first then the roof. An optional gun emplacement can be made in the center of roof, by cutting hole in new roof and emplacement parts made and glued in place. 

Step 8, Front doors were sectioned and refitted, and new rear doors made from the front doors of another humvee, which were recut and sectioned to fit.(if no doors are desired cut front doors at door edge and leave hinge in place to fill the void). 
 
Step 9, Paint the body using testors olive drab mold paint. Reassemble to chassis, and paint wheels olive drab also. Done.