Not bad, not bad...that's what I was thinking as well: some sort of "shock absorber" at the base, which is where you'll have the largest transfer of energy when the cab gets bonked around.
As for the removal of the "dummy tube", you might try a bit of lithium grease around the base of it before pouring in the rubber material. It would be easier if the tubes one encounters in Eurorack had "chimneys" that have the usual spring-type shock absorption, but that would make the tubes less "showy".