I'm gona throw out a word of warning here. There is a common problem with just about anything you can use for a "soft" type platen face.... that being as you grind, there is either a "wear" or a "crush" factor that you must deal with. What is that? In short, it means that with each grinding pass you make, the radius of the convex the platen face provides... changes, so it's constantly changing the radius, and you are constantly chasing your tail. How much and how fast it changes depends on belt speed and the amount of pressure you apply.
Probably the biggest offender in this realm is the graphite canvas that is sold as a "platen backer" material. Just steer clear of it. You can get MAYBE a 1/2 dozen grinding passes on it, before the graphite is gone, and the canvas is worn to the platen surface.
I'm pleased to hear folks talking felt. In my experience, felt is what I would call the best of the worst material you can choose for a platen face. I've tried layering, but for me it was too inconsistent...as one layer would wear into the next, there is a marked difference in the wear, and any "joint" between layers tends to make "worm tracks" on whatever you're grinding. I settled on a single piece of 1/2" thick "soft" felt from McMaster-Carr. That was before the rotary platen came along.
I've tried a bunch of different things for convex grinding over my career, and to be completely frank, nothing works well, or for very long, EXCEPT.... a rotary platen. It's a bummer that a rotary platen cost so much, but if you're intent is to do a significant amount of convex grinding, it will pay for itself in the frustration it prevents.
Here's the 1/2 felt glued to a platen face...
All of my grinder platens are setup so that I can easily change the "face plates".... just held by two 7/16" bolts.... (just came back and re-read my post... That should have said 1/4" bolts! It the heads that are 7/16"...GEEZ my brain is toast.) the following is the glass faced platen that is what I use about 99% of the time.... then the felt platen face mounted. Trust me, IF you choose to go the felt route, you want it sticking out that much.
As John mentioned below, my felt platen has been around forever.... but never gets used.