I understand the current plan, but let me run something by you.
If you have a heatsink for the ECU you don't have to mount the ECU to the firewall.
You can leave the ECU and heatsink floating so to speak. (as long as you have a ground wire for the ECU it does not have to be mounted to metal.)
You could also use the firewall as a heatsink and ground and mount the ECU with the thermal compound between them.
Just some possibilities that I didn't know if you had thought of yet.