I would also consider the "sandwich" method when it comes to 3090 GPUs. Grab yourself a heat sink, enough thermal pads to cover the surface area of the heat sink, and a fan or two. Strap them all together and sandwich them to the backplate to cool down those VRAM temps. Alternatively, you can grab one of these all-in-one heat sinks from Amazon. I am using it with the 3090 that's in my personal PC. It dropped temps by around 12-14 C with just the base package with the cheap pads on the heat sink.