I fashioned this backplate out of an led heatsink I found on Amazon. Just search 140mm x 300mm heatsink.
 
Had to do a lot of hand machining to get clearance for screws, and needed to countersink the holes so the factory screws would work. I used small nylon washers to make standoffs too. Super glued them together, then sanded them down until they measured the same as the factory standoffs. Then glued them to the backplate, and visually checked for component clearance using a backlight.
 
Next I cut a 1mm Der8aur pad to fit inside the factory cooler mounting bracket, and trimmed away holes for the large caps. Then cut another 1mm pad to lay over the top of that, and fit between the memory modules. Left the memory pads in place.
 
I slid the power limit and temp sliders to the max, (factory stock bios), punched in a conservative offset, and set the fans at 100%. Furmark was set at 5120x1440, and ran for 6 mins for each run. Any differences in power usage, clock speed and voltage, was done by the bios algorithm.
 
Up to 6C drop in temps.

I agree that I like the original heat sink but glad that yours works. I would keep the original around though in case you need to RMA.

Update: Finally got my hands on a 5950, and the AM4 mobo I use had the memory slots just a tiny bit closer to the gpu, so I had to trim several fins for clearance. Also, as you can see in the photo, I have a small section of fin removed, for access to the gpu lock clip. If I hadn't thought of removing that bit, the gpu might be permanently mounted to the motherboard.

Nice.
 
 
Looks crisp. I bet she runs.

Sorry, there is no going back to stock. The factory thermal pads basically crumble apart when you pull the backplate off. There was no access to thermal junction temps when I originally tested it either.

Here are 2 pics I scribbled on from the internet.
Layer one goes inside the tension plate, and layer 2 goes over layer 1, tension plate, etc. Just keep it inside the vram modules.
I also ended up replacing the vram pads with 2mm pads.

I ended up slapping a fan on it, directly over the back of the chip, and dropped an additional 18 C while mining with +1500 on the meme. So a total of 24 C drop vs factory.

https://ibb.co/XSgzJJb
https://ibb.co/M6WCZqs

vcjester
Here are 2 pics I scribbled on from the internet.
Layer one goes inside the tension plate, and layer 2 goes over layer 1, tension plate, etc. Just keep it inside the vram modules.
I also ended up replacing the vram pads with 2mm pads.

I ended up slapping a fan on it, directly over the back of the chip, and dropped an additional 18 C while mining with +1500 on the meme. So a total of 24 C drop vs factory.

https://ibb.co/XSgzJJb
https://ibb.co/M6WCZqs

Wow! Thanks! So do you know at this point with the fan what your actual vram temp
Is? Also, got a pic with that fan and the specs?