Thanks for the comments. Seems the common solution here is hose and valve down the side.
I did research Mann (the Provent maker) and they suggest either return oil to the sump, or vent the "cleaned" air to the atmosphere. Either way, there is minimal oil build-up in the catch can.
I think (but do not know) that blocking both oil drain and return air has some effect on the way the Provent works. Return air is under back pressure from crankcase pressure. More work to do