I sell soapstone in the marking grade and I am surprised you got the slots cut and in the pocket without it shattering. $13.00 per gross, by the way. There is a structural/architechal grade of soapstone that is a lot harder but no more durable. I have used brass, aluminum, bone, faux ivory, real ivory, carbon(about like soapstone)but a lot messier, copper, bare ebony, oil-soaked ebony, glass, teflon(high durometer), stainless, phenolic, mild steel and Oilite bronze. None of them are perfect. Never tried soapstone because it would seem impossible, to me, to sucsessfully cut it to shape and groove it. Didn't take long to file the slots, did it?
So far, my favorite is yellow brass with 6061-T6 aluminum right behind it.
Since you seem to be able to pull it off, why not try a bar of Ivory soap?
Sorry, Wes made me say that.