L shaped aluminum strip, Drill holes .8" apart (so 1.6" for a double) make sure to get the distance from the edge right, I used some spacers from my case to mark the distance.
Is there anything supporting those cards upright from the table or is it just a single L bracket along the top of the card brackets? I'm just wondering if you are still letting them be supported vertically by the slot, or if you have something else.