Download on makerworld for bambu studio print profile 

Magnetic GPU support bracket for MATX cases

Put a 8x2mm round magnet and the cover (rod) in the slot

Reupload from makerworld (https://makerworld.com/en/models/1468634-magnetic-matx-gpu-graphics-card-support#profileId-1532537)

I made it shorter and added a slot for a 8x2mm magnet.