If you can’t make it work with other programs you could do this with the boolean modifier in blender. I’ll give you a rough rundown of the necessary steps.

To cut the adapter to the right size you first create a cylinder that has the correct circumference and place it in the centre of the adapter. Then you select the adapter object and add a boolean modifier that should look like this.

You can then apply the modifier (by clicking on the small arrow in the upper right corner of the modifier card) and remove the cylinder again.

if you are happy with the result you can then move on to the next step. You could now merge the adapter again with a boolean modifier (although this time you have to select union) but if you want it to be recessed in the case you have to do another step first otherwise it would just fill the recess in the adapter model you have. You simply add another cylinder and scale it so it is slightly larger than the recess in the adapter and remove it from the case again by using a boolean modifier but with difference selected this time.

I hope this helps a bit. I know that if you don’t know blender this explanation won’t be enough to do this but if you have any questions then feel free to ask.

I think they both have their advantages. I found SolveSpace a lot more flexible with how you can create constraints. In FreeCAD constraining a line in proportion to another line is AFAIK impossible but at least not at all intuitive in FreeCAD but it’s quite easy in SolveSpace.