Hi!

This is a remix of https://www.thingiverse.com/thing:1471610 by rustybaker22 and https://www.thingiverse.com/thing:2945049 by DeemEnded.
Made from scratch in fusion 360.
Designed for smaller size cards. Mine are 67,5mm x 43,5mm.
Added front wall with cavity for finger.
No icons on the tray - they would not not visible when cards are inside.
Each tray should be printed in colour representing material (yellow for cereal, red for clay etc.)
Used the same magnets as in DeemEnded remix: spherical neodymium 6x2mm, like this: https://www.apexmagnets.com/6mm-x-2mm-disc-neodymium-rare-earth-magnet
Please note direction of each magnet. Schematic attached.

DeemEnded user has made explanatory picture you may see also in any further doubts.
Direction of magnets may be examined using one as referrence. Stick it to any surface and test each next one using it. It will always autorotate and attach to the sample with the same direction.
Magnets should fit to holes with little force. No glue. If you have difficulties in fitting, you may reduce "flow" parameter in Cura by 5-10%.

Edit 10.03.2019
Added sleeves to keep cards in tray after play.
Recommended printing method is 1 wall vase mode, spiralize outer contour, 4-5 bottom layers, 0,2mm layer height. PLA, ABS, maybe even TPU.
Print one and try. Should fit tight and get ok after few slides.
Resize in cura by 1-2 % if needed.

Two versions of stl: for 0,4mm and 0,5mm nozzle.