Sanding Concave Surfaces With DIY Foam Pad/Random Orbital

How to sand concave topographical surfaces without buying a fancy foam-padded attachment for your Random Orbital Sander

Cut Polystyrene into a circle the same shape as your pad

use spray adhesive to attach hook&loop paper with the fuzzy side out. 

attach sanding disk (sand paper out) to the other side of the foam.  

now you have a thickened piece of sand paper backed by foam that can follow concave and convex surfaces