You may not have to do O rings if you insulate it. I've heard of people using housing-grade fiberglass insulation to block against exhaust heat. I've sandwiched crushed aluminum pop cans between my muffler and rear belly pan and have had no problems. Crude I know, but also cheap and effective.
Also for oil changes and such consider what the EV1 used: access holes. Lots of them.
CCS's EV1 by
Tyler Linner, on Flickr
You could always build in little hinged doors too but that would be splitting hairs. Remember, in the grand scheme of things, a softball sized oil drain hole in your belly pan doesn't really matter that much.