The top part is fixed to the base by screws. I used some with 4mm outer diameter and 3mm inner diameter. The screws should cut their own thread but you can help them by carefully heating them with a lighter or similar.
Prints without Supports.
Since the base is too large for my limited z-axis travel capabilities (80mm) I cut the base in two. I uploaded both of these files and also the whole part for those with more travel or for those who want to cut it at another position.
After printing I just glued the two parts together.