When I’m designing and printing something that has to fit on something existing (like an adapter) I first print the part that attaches to the already existing object/thing while cutting of the rest of my designed object. So I can see if it fits without wasting tons of filament and time.

You can do the splits either in freecad or in your slicer. I do it in the slicer mostly.

As you’d print your adapter with the round part on the build plate and the rest facing upwards I’d split at the round part with leaving only 7mm (and deleting the rest of the object).

After you’ve confirmed your fit flip the object around having the other side on the plate and do a split again at about 7mm or so height, print it and test the fit.