Actually, there's a fairly easy solution. I didn't realize they actually made contour sanding blocks for balls (just recently started maintaining my own equipment) but I do a fair bit of automotive paint refinishing so I thought an abrasive holder would be a great idea.
All you need is a quart of fiberglass resin ($10-$12 at most any auto parts store) some saran wrap, and a nice sized dish sponge from Walmart. Lay out a piece of saran wrap, whip up about half a plastic beer cup of resin, dunk the sponge in the resin until it's fully saturated. Drop sponge flat on saran wrap. Cover sponge with a second piece of wrap. Set old dead bowling ball (you could use a good ball even, but why risk it) on top of sponge. The resin typically will set in an hour or so, depending on how much hardener you use. Remove ball, grind/saw/dremel until you get the shape you want, I ground mine off square. This will will give you a block perfect for use w/sandpaper. I also made a second one, except I placed a piece of scotchbrite between the ball and the top piece of wrap for a block matching the needed contour for thicker abrasives like scotchbrites or abralon. So for around $15 you have enough material to crank out all the blocks you, and probably all your friends will ever need.
I also attached the stiff half off velcro strips to the scotchbrite block,and hacked up one of those gel cell phone holder pads for the sandpaper block. None of that's necessary at any rate.
Yes it's a bit messy, but if you have a ball spinner already you're probably used to messes; besides it's cheap, and if you do it in a really small confined area I'm pretty sure you'll end up getting whacked out of your gourd on the fumes. Enjoy!
