I used to rappel a few years ago. At the time, I was about 275(heavier now but on my way back down), and I always made a swiss seat. I never once worried about it giving way.

Here is a link to a site showing how to tie one.

http://www.globalsecurity.org/milita.../ch6.htm#par13

Or you can try an industrial safety harness like this:

http://www.grainger.com/Grainger/items/4GE87

It is rated to 400lbs.

My advice would be to make the tree strap shirt enough so that if you do fall, you will not fall more than a foot or two.

Good luck on your future weight loss. Thanks for reminding me to hit the stairmaster tomorrow.