I plan on keeping it simple so my clueless mind doesn't get too overheated.

As of now I'm thinking 3/8th's plywood on the sides, masonite on top and epoxy paint all the way around to keep the thing watertight. However, I have a few questions for anyone interested in keeping me on the straight and narrow.
The trailer platform isn't as long as I'd like it to be as it is only 4 ft long. Since I'm 6'1" and plan on sleeping in this thing, I guess the floor ought to be at least 72 to 78 inches. If I bolt the plywood on the side of the trailer, ala the Cowper design, can I leave 15 inches of overhang front and rear to create my extra floor space? I don't reckon my feet are so heavy as to bust the connecting dowels on the rear of the trailer, but wondered if my sleeping on on the front portion, which will be hanging unsupported from the trailer frame might cause problems up there. Maybe I ought to stick extra dowels there to keep my punkin' head off the masonite?
Also, how watertight are these things? The unhappiest times I ever had in the military were sleeping in wet conditions. I'll be pulling this thing in the rain and likely sleeping in it in the rain and would like to stay relatively dry.

Also, has anyone ever used tongue and groove planking for walls? I checked out some cedar in 12 inch planks that was uber light and wondered if that might be a viable option. I made use toungue and groove for the 40"x48" HF deck and then try to cut my sides in a fashion to create the other 24"-30" inches of sleeping space I'm looking for.
Lastly...can I frame in the rear shelf now, enclose it with masonite and then come back later and cut my hatch lid in? I need to get it built QUICK. Staying in Baton Rouge last week I was lucky to find a motel that looks like it normally rents by the hour, but I didn't sleep too well because of all the crackhead noise coming through those carboard thin walls all night long. I'm hoping this trailer can be done fairly quickly and I plan on keeping it simple to help that objective along.
My wifes aunt is a far better carpenter than I am and she is coming down to help me build it as soon as I get the materials together. I may skip windows for now and just put a vent on top for the moment.
Any ideas? I'll try to take pics along the way. I'm hoping to end up with something similar to this, just not metal clad at the moment.
http://thundermax.tripod.com/Trailer.htm