I did this to my gaming PC but this can work for a mac aswell.

I was transfering my gaming PC hardware into a new case and when I took off the water block I broke the plastic clip for the mounting bracket for the water block.

Well to fix this I took off the rest of the plastic mount. I found two bolts with four nuts. I put electircal tape to the bottom of the motherboard around where the plastic mounts went though. This way the metal bolts won't hurt the runs on the circuits. I put the bolts though then went on the other side and used a nut on each bolt to hold it in place. Now I had to make a plate to hold the heatsink. I used some junk metal I found laying around the garage and cut it to 4 1/2 inches. Then I took the old plastic mount plate and used it as a templet and drilled holes to the size of the bolts I used as the mounts. I bored the holes a bit bigger since it would need a bit of play when mounting it. I put on the water block and the metal plate then used the other two nuts to secure the plate in place. It works perfectly and everything. Even looks like it was stock.
To live is to let die