I needed a shelf for my desk, so I made one.
Maybe you need one too.

It's a simple design with 4 separate parts that slide together.
It has a depth of 20 cm, and a width of 25 cm.
The parts are designed for printing on the A1, but you could probably scale it down for the A1 mini.

The sides are 1 cm thick, while the shelves are 0.5 cm, but you could probably be thinned down to 0.5 and 0.3 cm.