How to fill the gap between the GPU and the water block?

Question

GPGPU

Hello, I tried to install a water block on my Powercolor R9 280X video card and ran into a problem - the water block does not adjoin the chip tightly enough. The gap is very small, it is still not effective to fill it with thermal paste. I read about thermal pads, but they don’t seem to be used for such purposes, someone else uses copper plates.
What do you advise? What are the pros/cons of the methods?