they could be bevelled out (ground out) on the bottom.
Personally, I wouldn't waste my time trying to extend out the slate shelves. 95% of the balls that miss going into a pocket are missed by at least a half diamond anyway. Getting better at playing pool comes from practice...practice...practice....and not from making the pocket target smaller. If smaller targets and deeper shelves made the player, then all the top pros would have them, but they don't, in fact I know quite a few pros, and they practice on 4 1/2" pockets...but they practice on stance, stroke, position play....none of this has anything to do with tighter pockets or deeper pocket shelves.
I once was a player, so I know of what I speak, and it's not that I can't play anymore


Glen