J Burns wrote, on Sat, 19 Jul 2014 10:02:58 -0400:

I cut a 2-foot 2x4 and tacked some pieces of wooden
molding near one end to make a sort of picture frame the same size as my
stones. If I put a stone in the "box", it wouldn't slide.

That's interesting because I did have a problem with the stone
sliding around, although I think the small size of my stone
is another problem because the knife is much longer than the
stone.

So, I get *different* angles on various portions of the knife.