Cleaning the crumbly foam out is a good idea, while just slapping the back on afterwards is not. You have to have something covering the back of the board, or you risk shorting stuff out. Most of the MXRs came with a stiff paper bit to cover the back of the board, and I'll usually re-use that. Then I will also want something to cover the component side. I usually make up something that will wrap around the board (stopping it from knocking about inside the box and shorting out against the bottom plate), just to be safe.
Stupid question, but why did MXR go with foam when everyone else seems to have found a way to mount the board to the box in some way? Always seemed a little silly to me to have the board flopping around even if it is wrapped in foam.