Here's an idea for a robust microSD card holder with space for 18 cards. Since the design is so simple, it would be easy to adopt it for any number of cards. By mainly using 6 mm hardboard, the case should be able to withstand quite a hammering.
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Step 1: The Basics
The heart of the card holder is a piece of 12 mm plywood, with slots sawn for accommodating the microSD cards in an upright position (first photo). The card fits nicely into such a slot, with a few millimeters sticking out to grab it by.
The slotted plywood piece then gets 4 sides and a lid, everything made of 6 mm hardboard. The dimensions in the third photo should be self explanatory. On the left is the lid and on the right the card holder itself.
A piece of 3 mm hardboard is glued to the underside of the slotted plywood to keep the cards from falling out.
I used only glue to assembly the parts with, but if you want to go for maximum strength, the 6 mm hardboard should be thick enough to use small screws with the glue for a stronger construction. Just remember that all holes must be pre-drilled, because hardboard doesn't take kindly to being penetrated without the courtesy of an existing hole.
Step 2: Construction
We begin by cutting the 12 mm piece of plywood to size: 100 mm x 34 mm.
I've found the easiest way of cutting the slots is with a band saw (first photo). The kerf is usually also just wide enough (about 1 mm) to accommodate a microSD card.
The second photo shows the construction seen from one end on. Please refer to the previous step for all the dimensions.
First the bottom is glued onto the plywood's underside, and then the side pieces. Then the end pieces follow.
In the third photo you can see the almost finished product with the lid also cut out.
Notice the protruding lip of the lid in the last photo. This is for getting hold of it easily when opening the card holder.
Step 3: Construction Continued
In order not to damaged the paint finish during the last part of the card holder's construction, I decided to paint it at this stage. In the first photo you can see a layer of primer applied. For wood products like this, I use a PVA (water based) primer first, because it seals the wood better than an oil based one.
But before applying the final paint finish, there is still time to mark and drill holes for the hinges and closing mechanism (second and third photos).
Step 4: The Hinges
After applying the top coat to both the card holder and its lid, it is time for fitting the hinges. I used two small hinges I got at an arts supply shop.
As I did not have suitably small screws, I decided to use 30 mm x 2 mm nails, cut to about 6 mm (first photo). They were then glued through the hinges and into the pre-drilled holes with quick set epoxy glue. In the second photo you can see the hinges fixed in place.
If you don't have access to suitable hinges, glueing a strip of cloth in place of the usual hinges should also work quite well.
Step 5: The Shutting Mechanism
In line with the robust nature of the card holder, I wanted to use a secure method of keeping the lid shut when closed. Clips that could spring open on their own due to being jostled about didn't seem suitable.
Instead I resorted to a cheap and easy to make mechanism which keeps the lid shut under constant tension.
It consists out of a piece of rubber band, about 6 mm wide, two washers and two screws. The first photo shows the finished mechanism, and the second photo shows the parts.
The first screw is fixed to the lid through the smaller washer and through a hole in the one end of the rubber band, thus trapping the rubber band securely.
The other end of the rubber band is fixed to the larger washer by means of a piece of wire. The washer is then hooked over the second screw fitted to the lower side of the box's front panel when shutting the card holder (see first photo).
Step 6: Finished
All left to do now is to admire your work, and introduce your microSD cards to their new home.
Step 7: Organising Your Micro SD Cards
Having a lot of cards of course requires some kind of indexing system for the contents of the cards.
My solution is to mark each slot and the card which it houses with the same number (1 to 18 in this case). The contents of each card is then jotted down next to its number on a piece of paper, which is kept with the card holder.