Everyone probably already has a set of sanding blocks like these, but this is what I did tonight. These are for controlled sanding of ends and edges with minimal risk of rounding over the sharp corner.
I used a piece of 1 1/2" aluminum. On a 3/4" thick board, the ends and edges will support half of the sanding block surface, making the block less prone to tip over and round off the edge or end than a bigger sanding block. The rigid aluminum will also maintain flatness.