I just made one following vwone's writeup pretty closely. I couldn't afford to give up my sissy bar pad, so ended up creating one myself:
-1/2" ply as the internal backing
-some of that hardening spray foam that's used to fill holes around plumbing. sprayed on one side and then "shaped" with a razor blade after it hardened
-some all-weather vinyl I paid about $5 at the fabric store, hand-stitched to match the sissy bar pillon (this was the biggest PITA of the whole project)
-metal plate (can't remember the thickness but it is probably overkill - bought at local home depot and dremel'd to size) spraypainted black to attach to the bar that is mounted to the underside of the pillon, and to cover the internal 1/2" ply backing
I wish I would have taken some pictures of the process to create the new pad as I was doing it because I only spent about $30 total on the entire job and it was fairly painless. It doesn't look perfect, but good enough for me (I'm not entering any bike shows) and it definitely helps on longer rides.
