A stackable, horizontal test tube holder, originally intended for storing queen ants in test tubes. By printing multiple of these, you can place them on top of each other to conserve space.

The STL provided is for 100x12mm tubes (with 1mm slack), however the model can be customised for different sizes.

To store at higher density but with worse accessibility and a longer print time, see https://www.thingiverse.com/thing:4145165 .

EDIT: Thickened the walls as this was a little flimsy on my printer.