Besides building rods, the cork tape comes in handy on the platen too. I get 1"x1/8"x25' rolls from a fishing/rod building supply. The tape is peel-off sticky back and two pieces side by each fit perfectly on the platen. Make seperate platens out of aluminum angle. After a while the cork will dish in with heavy use.....don't peel off the cork, at this stage the dish gives a really nice shallow convex grind, if that's something you need to do. I also use the cork on some of my hand sanding bars.
Some ideas from someone who has never-ever used a soft backing:
Some of the parts houses (NAPA, Oreillys, Autozone- whatever you have in your area) sometimes have pads on the parts counter- with a slick plastic top over a dense foam pad. Maybe you could get them to donate their old ones instead of throwing them away? Seems they were kind of similar to the old mouse-pads.
Some high-dollar carpet pad is a fairly dense foam rubber- and how about the liner that goes down before laminate flooring?
My wife has a fancy little pad on the floor in front of the kitchen sink- like throw-rug, but it's 9/16" foam rubber with a skin on top- complete with rooster pictures!
Fred's felt suggestion is probably the best, but thought I'd throw out a few ideas, anyway.
I'm wondering- for what work do you guys find the soft backing most useful? Is it kind of in between using a flat platten and a slack belt?
Edit- Rudy posted while I was writing- and answered part of my question (thx).
I use the cork platen only if I'm going for a high mirror polish with belts from 1200 to 4500. The Trizacs work very well by themselves but the cork seems to squeeze some magic out of them.
The cork totally eliminates belt bump and gives me feedback as far as pressure against the platen. The discovery of how well it works for a shallow convex was a bonus after trying to finish flats on a blade without checking the cork first. Now I do it on purpose if I need a convex ground blade.
Of all the things I've tried over the years, felt has given me the best results. Check into Grainger, MSC, or McMaster-Carr. All have heavy duty felt, in a variety of densities. Personally I like the "hard" for my purposes.