So I've continued my attempts to create a 3D case for the device being designed. I still haven't completely decided how to place the two buttons but they will most likely go at the back. Here you can see the dongle with the OLED:

The OLED is (supposedly) positioned as low as it can (the crystal on the dongle limits the height. The buttons will go in the back, slightly increasing the length, but not by much.

The case so far looks something like this:

The idea is to have a cutout for the OLED. The case should be split in two parts for printing and assembly. However, the insides of this are really difficult to get right. I have to provide space for the boards and consider extra space for the components beneath the dongle. Moreover, it should be possible to slide the device in, which I don't think is really easy.

Since I'm just learning to do this I'm thinking I will simplify things initially. I only really need a case to hold the buttons, since I don't see a way of soldering them in place in the back. I will rethink this and try to simplify this stage and leave it for future improvement.