So here's the mod I did. I picked up a C-clip from Ace for 65 Cents in 1, and 1/8 diameter. Using a pipe wrench, I squeezed it until it could be pushed in. Using a screwdriver and a hammer, I tapped it down into place, and noticed that it left grooves in the metal as it seems to keep it's original shape. So I don't think it's going anywhere. Looking back on it I should have probably went with the foam route....it was a pain in the caboose to do.
+1 point for a more elegant solution (of a civilized age). I like that assembly to be snug. Not just for the aesthetics, but because unsecured sound board / battery modules have an annoying tendency to register swings as clashes. (sure you can tone down the sensitivity in the loader, but I like your solution.)