I first removed the aluminum face plate from the hard drive, allowing a view of the mysterious innards to passers by (there are likely a hidden screw or two beneath the sticker) .
I then used some 16 gauge, solid core, electrical wire and placed into the hard drives frame screw holes (cut enough to make your bends and and add for any ornamental/support curves).
I used different sets of holes for the 2 different orientations of the hard drive (vertical & horizontal) and how the cards were to be held or propped up (used double sided tape to hold against a fixed object, or use some more wire in the mounting holes on the side to create an easel).
You certainly could use a 3.5" hard drive, but its size might hinder the card takers ability to easily retrieve a card, or maybe not.
The solid core wire should allow for some ornamental twists to liven things up a bit, but that'll have to wait for some free time. So for now this is where mine are at.
I've only had to remove curious finger prints once from the platters, and I give a burst of compressed air every couple of weeks to blow out any settled dust. I get lots of compliments on the holder as it totally beats one of those office box shops plastic bendy things.