I really liked hackermagnet's original holder but my weird little printer always requires tweaks to make things size correctly. I get particularly bad shrinkage on small holes, so I knew I would have to tweak all the screw and nut stuff. In the course of making the adjustments I needed, I ended up cleaning up some inconsistencies in the .scad code, exposing more parameters in a cleaner way in the customizer, and adding a few improvements:

• Corrected customizer syntax and split the parameters into separate sections for Dimensions and Options
• Resized micro SD card slots (they were a bit tight compared to the SD)
• Made SD and uSD card dimensions adjustable in customizer
• Increased default screw hole sizes to allow for shrinkage
• Increased default magnet sizes to take 5mm x 3mm magnets (closest I had on hand)
• Increased spacer size to accommodate larger screw hole
• Added build_plate library to ensure makeability on my small printer
• Made magnets optional (set magnets to 0 to eliminate them)
• Made top/bottom text a parameter (leave blank for none)
• Parameterized screw head and nut diameters
• Made screw hole and magnet positions dynamic based on hole size
• Parts always build centered on build plate
• Moved top's magnets to inside of finished assembly for a cleaner look

I used sliders for a lot of the parameters in order to control the range and step size so the model wouldn't break.

For assembly I used black button head screws and hex nuts, which blend in nicely with the Geeetech black PLA I printed with.