Computer mouse pad is way too dense. Newton's 3rd law. The mirror bounces back off the more "bouncy" denser foam, and that is harsh on the hinges and actuation linkage. I had fair luck with the foam that HO gauge model railroaders buy at the hobby shop to use under their tracks. I slit this material as needed to customize a new bumper foam. Remember that the shiny face of the foam has to be slit off to reveal the actual bubbly interior. You do not want the mirror slapping this surface, as it has no "give". Because this foam is denser than the original, I find I can compensate somewhat by making the width fore-and-aft narrower. This allows me to retain the height of the foam so that the mirror strikes it in the correct last thousandths of an inch in its upward travel. Be very careful not to make foam height too high, lest the mirror will not be allowed to fully travel. Causes jamming. If the mirror is allowed to over-travel, it makes the hinges "sloppy", resulting in malpositioning of the mirror in the down position for focusing.
To slit the foam, lightly spray-glue a piece of chipboard, stick down your foam, use a ruler to cut fore-and-aft width. Pull up the excess. Then use "rails" you have constructed out of paper and chipboard to put on either side of your foam strip stuck to the chipboard. "Trap" the foam strip between these rails. Lay brand new razor blade down flat across the rails, and begin slicing the length of the foam with a gentle sawing motion. Cut the foam to proper length and attach to the front mirror-box flange. (with tweezers) Do not allow glue to get on the groundglass. You'll never get it clean.