Ok, I think these fixtures will work: Amazon.com
Out of the box the thickness of the head was too much to fix in the dimple, but I sanded them down on a belt sander until they fit with the default o-rings that came with the Pro. I haven’t gotten to the point of filling this to verify that they’re water tight but I don’t see any reason it shouldn’t be (worst case I’ll change o-rings or use plumber’s putty or something along those lines.