Those of us that have been around a long time remember that "old school" server pedestal (and rack) cases had what were called "card guides" that supported and separated the long cards, so that they did not sag and kept a uniform space between them (allowing cables to be connected to the cards without interference), and made sure the older, thicker cards did not touch ).
The better of these guides covered all the slots in the case and had adjustable "fingers" (not all cards were the same height) that held the ends of the cards.
The old 10.5u pedestal/rack case I use for my workstation is set up this way.
The card guide covers all 9 slots, is attached to the removable motherboard tray itself (thus supporting the cards in any position during assembly), and the plastic (non-conductive) guide fingers have almost 1.5" of adjustment (works for the tallest or shortest (LP) cards) and fit over the circuit boards of my 7970s at the far (power connector) end of the cards.
I couldn't find an exact picture of an example, but these will give you an idea -
http://www.plinkusa.net/products/4088-20.jpg
http://images.highspeedbackbone.net/itemDetails/Vision/callouts/V133-2064-out-la.jpg
In my case the motherboard tray has multiple tapped holes allowing the card guide to be moved over half the length of the card (front to back, thus allowing for best positioning).
And while the fingers in the above pictures are dependent on "spring pressure", those in my case are attached to the side of the bracket with a machine screw and the fingers are slotted for vertical adjustment, positively keeping the card in it's proper position in the slot regardless of the position of the MB tray.
If I didn't have this case with card guide, I'd use a thin piece of Al. angle for a bracket and some thin plexi/poly for fingers and fab a card guide.
With all the case modding that goes on, you'd think someone would make a "universal fit" version of those "old school" card guides :whiste: