I personally use copper ramsinks like the Alphacool 17427 or Swiftech MC14 applied directly to the controller with thermal tape. If it must be full-drive and/or low profile, icepc's pure copper heatsink - 2mm or 4mm depending on clearance - is ideal.

Search 'backplate heatsink' on Amazon, eBay, or Aliexpress and you'll find there's lots of options pre-made for this. Some are hunks of finned aluminum with and without fans, some are copper plates with heat pipes soldered to them going to fin stacks you can mount fans to. Depending how much room you have around the card you could probably just thermal tape a low profile heatsink/fan to the back of the card after you put better pads between the backplate and RAM modules and get effective cooling. If you're going to remove the backplate entirely be sure to note any other parts it contacts and put heat sinks on them as well. Alphacool makes some nice looking copper heatsinks in various sizes with a lot of surface area that would also work pretty well but at a higher cost because copper and brand name. You'll need thermal adhesive or thermal tape to stick them to the RAM modules. Ram is 14x12mm so these 14mm heat sinks should work, you may need to trim one. Be sure to note the fin direction and airflow direction when installing for best performance.

https://www.amazon.com/Alphacool-Copper-Heatsinks-14mm-10-Pack/dp/B077VM8FJW/