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My comment doesn't completely apply to your question, as you obviously want a hard non-give surface to apply the the chromium oxide to. However, I am quite happy using balsa wood for my chromium oxide. (I cut a 3 inch by 3 foot strip, bought from a hobby store, in half, as is; don't attach it to anything else at the moment.)
I like balsa for my chromium oxide, compared to something more firm. I use a very light touch, and the slight sponginess of the balsa seems to give my edge a more uniform polish than when I used harder substances (like plexiglass.)
I already use balsa for my diamonds, though I don't currently own any half micron chromium oxide. It does work well, and I like that it has some give too it, but not too much.