1). Masking tape down the road may cause some "issues" and could damage the cards.

2). I might suggest taking the time to cut a "matte" for each individual card to be held in the display with a little space between each key. This will serve two purposes. a) you won't need to tape each card if matted correctly and b). it will create a small space between the "glass" and the key. Make sure to use "acid free" matte and backer boards and, glass I think would be preferred to plexi-glass for long term display.

Just my 2 cents worth, take it or leave it.