I got a solution hold on
Use fishing line-
You'll need like 6-9 pound clear line, a swivel, and a snap (or just two snaps)
tie a very short length of line to the op of the case, and a put a swivel (or second snap) on that line, just so that it's out of sight.
Tie snap to longer length of line, and run it down to the gpu
Unscrew a screw from the back of the card slightly, loop some fishing line to it, re-tighten it. Make sure to pull the line nice and tight- if you have trouble, maybe attach the short length last then.
This fishing line will hold the card, be detachable via the snap, and be very low key/hard to see.
^Swivel snap combo
This can all be accomplished for cheap via a trip down to the local bait and tackle.