Thanks, Mike.
I checked with a buddy who is a journeyman auto body technician. He's confident that the JB Weld should hold no problem.
The heatsink isn't very heavy. So, I'm going to try out my plan as it is. In the event of a failure, I'll put screws through the lid. If I need to do that, I'm going to make another modification to the lid as well (cut a hole in it for a 120mm cooling fan). And, in the event of a failure, I'll be sure to document it online: so no one else attempts it.
