Here's a DIY option:
DIY Contour Sanding Block
All you need is a quart of fiberglass resin, some saran wrap and a nice sized dish sponge.
1- In a well ventilated area, Lay out a piece of saran wrap.
2- Then mix up about half a plastic beer cup of fiberglass resin.
(the amount of resin you mix would depend on the size sponge)
3- Once mixed dunk the sponge in the resin until it's fully saturated.
4- Then drop the sponge flat on the saran wrap you layed out.
5- Next cover the sponge with a second piece of wrap.
6- Then set a old bowling ball (you could use a good ball even, but why risk it) on top of the saran wrap covered sponge.
Now let it sit till the resin hardens, The resin typically will set in an hour or so, depending on how much hardener you use. Once hard remove the ball.
Now you can grind/saw/dremel and smooth your block until you get the shape you want. This will will give you a block perfect for use w/sandpaper.
You can also make a second one, except place a piece of scotchbrite between the ball and the top piece of saran wrap for a block matching the needed contour of the thicker abrasives like scotchbrites or abralon.
You can also add some velcro strips (the hook part) to the block, so the abralon will attach to it or if you use hook and loop sandpaper.
