HEI in Mopar ECU casing

I'm not attaching the heatsink and module to the mopar box. I am attaching the aluminum heatsink with the module to the firewall, then the hollowed out mopar box just bolts over it to cover it. now im trying to think of how to attach the aluminum heatsink to the firewall without drilling holes in the firewall. Thought about just jb welding the heatsink to the firewall but then my idea of using thermal paste between the heatsink and firewall can't happen..