You could /possibly/ use something permanent like AS Alumina to attach the heatsink to the GPU, and judicious use of cable ties to add more support, but unless budget it a serious constraint dropping twenny quids on a proper 3rd party GPU cooler is a much, much better option. Even then, with the greater risk of burning out your card due to an incorrect fitting I wouldn't bother, and I'm used to bodging together fixes! Bear in mind most GPU coolers are designed to give airflow to not only the GPU but also VRAM and voltage regulators, and a stock CPU cooler wouldn't do this.
For a zero-budget solution, try airdusting/vacuuming your gfx cards fans and vents, and have a look at your case fans to see if you can redirect more airflow to the card to keep it cooler in the first place.