By the end of your post I was searching for lightweight frisbee's
I hadn't twigged that thenorthwind's aerobie superd[redacted by compression sack] is a disc and not a ring. Looks ideal in fact.
As for whittling one...I ended up in A&E on a Friday night (not recommended) after trying to make a simple spoon and the blade snapping shut on my finger. Fascinating watching the surgeon peel back the skin to stitch the tendon back together, a bit like that scene in the Terminator (not for the squeamish, and on that note apologies to anyone offended by the language in the clip I posted on the whisky thread).
I think, the Flextail 1.2 that Mr Climber showed a review of (picture showcasing). Less the ability to pump the mat whilst it's inside the bivy, but more the way it sucks the air out. So it can be neatly rolled away quickly without worrying about removing air by hand.