Or, upgrade to a Jobo and use add a nitrogen tank to put an inert layer of gas (nitrogen) in the tank as the developer works. One time expense of a tank, regulator, and some simple tygon tubing and all you ever have to do is get your tank filled up occasionally. As long as you remember to turn the gas off after the developer step, even a small tank will last a long time.
Not only will you be able to get plus development levels that are not possible with PMK (or Pyrocat, or WDd2D, or [filll in the blank with your favorite pyro based developer]) in rotary tanks that are open to the air, but you will even get shorter development times as the developer holds it's activity when the nitrogen keeps the oxygen/air away.
I haven't used this with Rollo, but I have with PMK, and Pyrocat, and WD2H.