A simple remix of a great work from jef4.

His design is excellent. But I make some improvements.
1) I cancelled plugs because covers will be pushed toward each other if I opened trays too hard.
2) I cancelled plugs to simplify printing process.
3) I modify the profile of the tray so that the tray will not be dropped off.

To build the SD holder, you need to print 2 covers, 1 SD tray, 1 SD/mini SD tray, 1 SD/micro SD tray, and 2 screws of M3.5 X 6mm.