Well, I used the 'Heatsink module for 1" ID tubes' to modify an Initiate V2 and a Liberator V2.
On the Initiate I used the original set screw to 'fix' the heatsink and used the 'top hole' of the Initiate for a new set screw
-> The blade now is only 1" in, but as my son is swinging the saber around without heavy hitting (or dueling) it's OK for now.
On the Liberator I applied a 'non-reversible' method: I used a steel nail and hammer (through the switch hole) to create a little rim which prevents the heatsink from sliding too much into the hilt (or even pushing against the switch). As long as the blade is in, there's no rattling, all is fixed and fine