On remote mounted [ Dry Sump tank@ one end/pump@ other of car ] I use 2.5mm/3.0mm wall alloy tube with either a barb or AN fitting machined/welded on, remember these lines are not under any significant pressure though. I dont use Air/Oil Coolers if possible, that means the water/oil exchanger cam be mounted close to the Water Pump intake & after all these years have come to the conclusion that if you need an accusump you really need a dry sump setup at this point in time. Not sure who had an extruded alloy product, finned hard line that was threaded @ each end for AN fittings, it looked ideal to act as a cooler as well if you can get some airflow around it. In the engine bay its hard to beat flexible hose lines though.
EDIT, found finned aluminum cooling line, might be available from Earl's.. its -12, threaded 3/4" npt @ both ends, slot milled/extruded to fit over bolt head for simple mount & 4 foot long, approx 1.75" OD of fin's..