When I was rebuilding my Amerigo, I found the patent for the snap n nap....it doesn't give you exact dimensions you would need, but cool old drawings showing the design concept....may give you some ideas... try making a scaled version to see if you can get the geometry correct...
Don't worry about the large opening, box it with 2X4's and you'll be fine.....the rest of the interior (bathroom door frames, cabinets, floor. etc.) all lock the structure together to make a strong camper structure. You'll need more than a door opening to get the leverage needed to close the rear bed...opening is easier and done from out side the camper...just some tips I'd thought i would share..
Good Luck Joe!
Patent Link
