Owen Lowe said:

I ordered a sheet of this 1/16"x18"x21", $7.25 stuff:
http://www.aftosa.com/neos.html
a few weeks ago and am very pleased with it. The stickum is tenacious!

I used Buna-N (Nitrile) rather than neoprene, for its resistance to
abrasion and chemicals. 3' x3' x 1/8" sheet of Duro40 for about $20.
Works pretty well, soft "sticky" surface, and keeps the workpiece
stable. It recovers from indentation well, and as long as the rim is
reasonably flat, grips tenaciously. For more flawed rims, thicker
material could be used.

http://www1.mscdirect.com/CGI/NNSRIT?PMAKA=31935117

It is not self-adhesive, however. Not really a problem AFAIK.
They do sell adhesive versions - at a price.

Neoprene, due to its lower tensile strength, could be considered by
some a safety factor against catching on the material should you
decide to cut close to the mounting point.

In the end, any material that works is better than no vacuum chuck.

FWIW

Greg G.