I have used silicone rubber to fix my secondary, and it works quite OK. Use just 3 "dabs" to fix the mirror (DON'T use whole surface), otherwise you'll have hell of a time trying to get it off ! I used 3 thickish carboard insers to keep mirror away from a metal support while silicon sets. After it sets, just try to pull the mirror off (in the case that RTV hasn't properly adhered - grease, etc). It it doesn't peel away, it will be quite securely fixed. This way you really gain two ways : - less obstruction (no extra support around secondary) - mirror is not in the contact with metal (less thermal effects, and less chance for mirror to get pinched by shrinking cold metal)
To get it off, use knife blade to cut away the silicon, and then use solvent to remove the residual silicon bits. If you do this for the purpose of realuminizing the secondary, make sure that ALL silicon is removed. Otherwise it will outgas in the chamber, and your mirror won't be well coated (not to mention nasty calls from aluminizing mob!).
Bratislav