Put some crap in there.
Just a note: I have the DEA solid mounts, the engine mount seems VERY solid yet there still is a cavity beneath the main section of rubber and the bottom metal "cap".
Its all filled now, going to wait a week or two. Most of these products claim a one week to fully cure.
I'm sure they dont count on their product being crammed into a 2x3x1" thick block though.