This is a holder for the SodaStream Glass-Type Bottles.
The bottles can be inserted with or without a cap, empty or filled. They are held in place by a recessed surface that fits the bottle necks shape (specific to the glass bottle types as seen on the pictures).
The holder is designed to be mounted underneath a shelf board or kitchen cupboard using two screws (I used 3.5x35mm). The design ensures a proper distance to the wall. Just hold it against the wall while mounting it. The distance between separate holders has to be adjusted as required (it should be at least 25mm to end up with an appropriate gap between the bottles).
Update: increased the hole size a little since I had problems with some screws (now screws with a head diameter up to 8mm should fit)
I printed it with 1mm shell thickness - feels robust enough to me