Looking for advice with displaying graded cards

[Joshchisox08]

I finally have a place of my own and have walls where I can hang up the display cases that I bought off of eBay months ago.

The problem is that the cards fall forward when upright on the wall. The solution (for the time being is I put something behind the cases which you can see makes the bottom stick out so the cards are resting back on the back panels of the case.

I don't really like the way it looks. I'm looking for suggestions on how to handle this situation. I'm pretty pissed off as those cases where pretty expensive considering the material which probably cost about $10 if that to make them, doesn't keep the cards snug and secure to display on a wall (ISN'T THAT THE PURPOSE??????????)

I obviously don't want to glue, or put a adhesive that would destroy the slabs nor the crappy wood paneling that they're on. I had thought perhaps maybe something similar to painters tape (but that may not be strong enough to hold)??

Looking and open to suggestions so I can have the cards stay in place and not have to worry about them all tumbling down if I want to swap out a card to display.

Thanks in advance and Cheers,
Josh