Prior to owning an MFK, I made my own flush trimming jig for one of my old routers (A Dewalt 610...). Fiddly height adjustment was my biggest complaint. I was going to replace it with a newer router, then discovered the MFK.
Anyway, I basically made a larger version of the 'other' horizontal base that you can buy for the MFK (it's the non-2 base kit, that is meant to trim with the motor in the vertical position). I just used any flat bottom bit. Usually, it was a 1/2" two flute design, but I liked using a couple other bits as well.... Always considered a bowl bit, but never got around to it... thinking the rounded profile would help make a smoother transition.
Anyway, it works fine, but the key to have a two step base. You could probably just cut a piece of 1/4" MDF and screw it to the vertical base fo the MFK?