I bought one online made by Wilwood. Was told it had a sealed cap but when it arrived I discovered it had a baffle and vents directly to the atmosphere.
I found a Dorman accordian type cap gasket that fit with a minor trim so now it's a sealed reservior, although there's only enough "accordians" to compensate for a partial fluid drop.
Filled the MC cap with lead, tapped in a couple set screws to plug the old vent holes then tapped a brass hose barb in the top.
Still sitting on the bench so I can't say how good it's going to work.