I found some stuff that seems to make a nice butt pad if you don't want to use the standard Riffe piece, or flip-flop section.
The material is SBR rubber, which is the stuff that they make laboratory test tube stoppers out of. It seems to be a good balance of soft, tough, and workable. Best of all, it is relatively inexpensive. These laboratory stoppers are available in quite large sizes. I used the third from the largest size that I found in McMaster, which cost about $7. They have a slight taper, and this one is 3" diameter at the small end, 3-1/2" diameter at the large end, and 1-1/2" thick. I think that they are available through about 4-1/2" diameter.
It saws easily, and can be sculpted quickly and neatly with a belt sander. A dremel will work for finer details. I mounted the pad by drilling four flat bottomed holes with a 3/8" Forstner bit, which you can get at Home Depot for about $4. I used 2" long, #8 stainless screws, and put a #6 finishing washer under the head of each screw. The finishing washer works great, because it bites out on the outside edge, and doesn't tend to pull through the rubber the way a flat washer does. I got the finishing washers at West Marine for about $0.09 each. The four screw mount feels very secure, and recessing the screw heads 1/4" was plenty deep to accomodate the compression of the rubber against my chest.
I still need to shape it a little more for looks, but I have to say it is supremely comfortable. I think this is just the ticket for either hip or chest loading. I'll never use sandal sole again.
Sorry for the large pics Dan. I mashed the wrong button. Feel free to resize if you want.