Hi Rambo,
Welcome to T&TTT.
Let see if I can give you some ideas on your questions...
1) The best way to seal the hatch is with some kind of weather stripping. But any kind of seal that compresses should do the job. You can either put your weather stripping on the hatch, or on the sidewalls where the hatch comes down. Most people use aluminum angle to keep water from going in sideways. Check out this page for hatch molding
http://www.outbackteardrop.com/body5.html
2) Well of course you can make your own axle. If you're good with steel tubing, flanges, welding, etc there's nothing stopping you. You'll have to add springs too. And if you are going off road, or if the roads are bad there, you'll want to add gas shocks too. Check out this page for gas shocks...
http://www.outbackteardrop.com/body8.html
3) Wood is plenty strong for the body of the teardrop. This outback teardrop uses a strong steel chassis, with a wooden body, and after many miles and many trips in the desert, the only problem was a bent axle shaft. Larry replaced his 2000# axle with a 3500# axle. Here's his information on the new axle...
http://www.outbackteardrop.com/body9.html?
You could find out on the web how thick the axle tube is, and then make yours the same thickness!
Larry has a lot of information on his page for a rugged offroad teardrop. Thanks Larry!
http://www.outbackteardrop.com
The quality is remembered long after the price is forgotten, so build your teardrop with the best materials...