It is a simple SD card holder which has attachment slots so you can add more SD card holders to it. When attaching the holders together you may need to apply a little bit of force. Do not worry as the force required to attach them shouldn't break the holders. You may also need to apply a little bit of force when inserting the sd cards for the first time, the force required to do this also shouldn't break anything. 

I may make updates to the design so that it is slimmer or to make inserting SD cards easier so make sure you check in on my profile to see if I have Improved anything

(UPDATED) I have updated the SD card holder to be thinner AND it fits SD cards better than before. You can know fit 2(ish) SD card holder V2s in the space that 1 of the V1s needed as demonstrated in the image below. The attachment pins have also been reduced in size and re-located. The V2s are a bit longer so I will keep the V1s up if you wish to use them.

I printed this model with a 0.4mm nozzle