Just getting stuck in and the first version seems to have paid off:
3D print wise I need to add in supports for that back edge, but the following video shows it all working as expected,
The 'basic' dimensions are as follows:
Essentially a 0.3 mm clearance, the lateral movement visible in the video was due to 0.5 mm clearance top and bottom.
I had attempted to put an o-ring grove in on the main part, but in hindsight I believe this is better built in to the clip.
Also the 0.3 mm clearance along the main mating face might be better at 0.1 mm to ensure any o-ring is correctly compressed.
Pressing the clip in is a bit difficult and requires the clip to push back slightly. To fix this the corner on the bottom edge of the retaining part needs to be angled.