My current solution for a "graphics card prop", is a 3/4" x 1.5" x h" piece of wood where, h is "just the right length". A rectangular box has been cut from the top, leaving a supporting shape (like an "L"), to cradle the corner of the card. The computer case holds one end very securely, but the other end hangs; hence the need for a support like this. What I described above took me about 10 minutes from design to implementation and has been serving just fine.
But I am planning to get a new (used) graphics card soon, and thought it appropriate to try to improve on my design! ; ) Incidentally, my design only works provided you don't move the computer, but this is not a significant problem for me.
Here is Amazon's solution:
I'm been thinking about this for a few minutes, and I have an idea or two, but I thought it might be fun to give us a collective design project--sort of like they do at MIT! : )
Guidelines are "Fast, simple and cheap", with readily available materials! Have fun! Don't overthink it! "Adjustability" with the turn of a screw would be a desirable feature. Something you can have confidence in (without the need to check it) would be nice too. If you are really successful you can probably make hundreds of dollars at Amazon! Besides for notoriety here, you'll have to "cash-in" there! : ) I know this is not everyone's cup of tea! Obviously, if you don't want to think about it, then don't do it!