You could push the gpu up with your finger till the sag isnt there and measure the distance from the GPU at that position till the bottom of the case. Then go to a carpenter shop or a metal shop and ask them to make you a T shape piece of material with the height being the measurement and the "T" being the base so it can sit on the case comfortably , just put it under and it should work , You could also add a small silicon strip at the top to prevent scratches and also paint it black to make it match the case!