I normally carry a 12' piece of webbing that I can tie into a Swiss seat (lower part) in an emergency. I have a 31" waist, and my waist is smaller than my hips, so this length of 15mm webbing works nicely. When there are bigger people, I normally just tie a 3-point diaper sling, with 7-9' from the same piece of webbing, as here:
http://cdn-www.trails.com/imagecache/ar ... 95x195.png

...and just tension the belay rope enough to keep it from falling down.

I was recently faced with tying a diaper sling (same max 12') for someone who was in decent condition, yet had enough of a paunch that the waist could not be cinched tight enough to keep the diaper from falling off. I tied the knot so there was only about 9' for the diaper, and it seemed pretty tight when the guy was standing up. The problem was that when he bent over, there was a redistribution of webbing among the three bights that clip through the carabiner, which always resulted in the waist area becoming too loose.

It seems like there must be some way to modify the set-up, still using just 7-9 feet of the webbing, so the waist band does not redistribute and get looser.

Hold the waist tight by pinching it right next to the 'biner. Wrap the pinch point with duct tape.

Needn't be very strong, just keep the webbing from slipping through the 'biner when unweighted.

I have used a swami belt made from 3 to 4 wraps of 1" tubular webbing tied with a water knot with long tails and a second double length sling with a single twist in the middle forming two loops. The legs go through each loop and a carabiner connects the twist to the swami.

Before my wife lost a sh!tload of weight, I carried a 23' length of tubular webbing for tying a swiss seat (as I learned as a young man) which apparently is a swiss seat combined with a swami belt. See http://www.itstactical.com/skillcom/knots/knot-of-the-week-hasty-webbing-harness/ for Hasty Webbing Harness Method 1. Any excess webbing can be wrapped around the waist, similar to a swami belt.

If she wasn't climbing, I had another really long sling!

Thanks guys, I'm still trying! I just figured out a way that uses less webbing than a Swiss seat, stays secure, and has leg loops to put weight on the pelvis in a fall. The disadvantage is that I will have to get uncomfortably close to tie it on other people.