How to build a Camping trailer, and riding across country with it
I haven't built a bike trailer but used a B.O.B. Yak for a trip from the UK down through Europe.  It was a great trailer and we were able to fit a large 2-man tent in there plus other stuff.  We also had front and back panniers.  

See here:  http://www.bobgear.com/bike-trailers/yak

I now use a Burley D'Lite with 2 toddlers in it.  Even though you could fit more stuff in it than the B.O.B. I would be wary of getting the centre of gravity too high.  

See here: http://www.burley.com/page_308/dlite-p308

The thing to bear in mind is that these companies have researched the handling, aerodynamics, load bearing, etc which is something that you'd need to consider.

For touring I'd say the Yak is the way to go as it is designed and built specifically for this use.

Edit: Also, bought trailers come complete with the things you need to attach it to your bike.