Is it possible to make a hermetic sealed 3D Printed case?
A few thoughts that might help...

Material:


ABS can be vapor smoothed with Acetone which results in the layers sort of "melting" together to form a smoother, and less porous surface.
Other plastics can be smoothed with compatible solvents, but I've not tried solvent smoothing with anything other than ABS.  Be careful if you try.


Print Method:


Consider slightly higher print temps to increase layer adhesion.  You'll likely have to compensate with extra retraction to avoid excessive stringing.
Consider more perimeter layers and more top/bottom layers.
The CF materials are stiffened with chopped CF strands...I think it's a stretch to call them "reinforced" unless you happen to have a Markforged printer or similar.


Sealants: This is probably your best bet.


Epoxy: Generally considered effective for producing hermetically sealed containers.  Dip or brush on.  Mind your VOC's and pay attention to working time.
Plasti-dip or similar sealants: These may be good enough for your application and result in a rubbery coating over your part.  Good for water sealing, and may be close enough to hermetically sealed for your needs.


Design:


To mechanically seal the opening, there are many options depending on your requirements.  O-rings, gaskets, etc.  If you use a rubberized dip, you may be able to skip the gasket.  You could install a few threaded inserts around the perimeter of the opening, put in the screws, then dip.  After drying, you slice around the screw and remove it (this just keeps the coating out of your threads)  Dip the cover as well.  Then when you screw on the cover, it will provide a water-tight seal.  To help make a good seal, apply a silicone grease to the mating surface.


I hope this helps.  :-)