Gluing silicon heater to aluminium
I suggest not gluing it. Starting from the top, make a sandwich this way:
Aluminium with holes for bolts - Silicone heater - Thin cork (the one from IKEA, 2 mm thick for office desks is fine) - Thin plywood with holes for bolts (or other stiff material holding at least 60°C)
This way you use the aluminium and the plywood to keep the silicone heater well in contact with the aluminium, and the cork insulates so that less heat is lost on the bottom side.
Also, cork is fire-retardant.
If the heater fails replacing it is simple.
Also, you can and should cut away from the cork some space for a thermal non resettable fuse at 180°C to cut power if the heater overheats.
In my case I should have used one more bolt, as you can see in the photo.