The best sanding block is one that matches the shape to be sanded. So recently when I had to sand some cove molding, I made some custom sanding blocks from 1½"-thick foam insulation.
First but the foam block against the molding and trace the profile, as shown in the top photo at right.
SHAPE THE BLOCK
Next, the center photo shows the profile being roughed out on the band saw. (You can also use a hand saw or file to create the rough shape.)To smooth out the shape, place some adhesive-backed sandpaper across the workpiece with the cove and rub the block across the sandpaper (bottom right photo)
SAND THE COVE
Finally, to use the sanding block, remove the sandpaper from the workpiece and stick a new piece on the curved face of the sanding block, as in the above photo.
Ted's Woodworking - Guide eBook