I own a machine shop so the round 1/4 deep groove is a piece of cake, but I might be willing to make some if you supply the material blanks.....
Not overnight mind you, but I will make a few pairs...
as for adding material, you just remove the outlet port and pour your media in,
I have already tried blasting media, and it moves and floats funny, but I am trying pool sand this weekend
Those are also "instant" push in fittings, very quick and clean.....
there is also room on the inlet for a small round screen to be cut and placed in case of a back siphon if no check valves are used, as I am not using them.....