Konihlav: Those are good suggestions, but I doubt if I will have enough tilt plate thickness to include both the bayonet and a filter. But a short bit of an M48 thread on the side facing the camera might allow a filter to protrude into the camera opening. I think, though, that the autofocus electrical contacts would interfere.
The Sigma 14-24 mm f/2.8 DG DN lens actually has a rear clip-in filter provision, which is nice, but that is probably the only lens ever to do so. But my other Sigma ART lenses for EF mount would couple via Sigma's MC-21 mount converter, which is 24 mm deep. Perhaps there is space for a filter inside there.
I might need two L mount plates, one thicker by 1/3rd the filter thickness.
I usually mount the camera lens in a scope ring, and let the camera hang off the lens; see my post:
FWIW, I just got an email announcing that Agena Astro is now taking pre-orders for the ASI6200, for delivery in "late January". I placed my order for the OSC version.
All the best,