I did the same thing to my t-300 with a 26' box. it cost me a grand and took me a week.it was sweet, alot better constructed and insulated than the crap that is out here now.it had a separate entrance/screen door on the right front corner,windows on the left wall.the opening has to be at least 36" square.i used 4x4's set on 12" centers insulated with 3/4 plywood for the floor.walls were 2x4's on 12" centers,heavily insulated, lag-bolted to the floor.i ran romex just like in the house, with outlets and ceiling lights, etc for use with the generator.i also ran the wiring off of the batteries for lights and things like that for going down the road.the sleeper boot, i took the rear window out of the t-300 and extended that opening down to a total of 36 inches.then i cut the front box wall on the same location to match that of the cab.i cant remember where i got the rubbre from, but i made 2 metal frames that went around the perimiter of the opening and screwed it to the walls after applying liquid nails to everything.the interior got covered over with rubber-backed carpet runner from home depot, and framed in for the finish. i made sure NOT to make the cab/sleeper join together firmly because they flex between each other.it is the quietest and warmest/coolest sleeper i have ever had.my freind did the same thing, only making his 10 feet out of a 30 foot box.they are still both on the road, after 5 years, and no leaks.i will do this again, in the future.