or you could use something like Trinity's Bomb gllue (for tire inserts) to gllue the insert to the wheel as well as the tire carcass if you want. You could also use slightly wider rims or inch ups to decrease the airgap.
Having an airgap in the tires is a drivers preference. i like a little, but if ihave no air gap, the inserts seem to heat faster.
For my sorex 36's, I like to buy 26mm standard rims and HPI 24mm inserts, this gives just a little less air gap then usual.