Adapting a us base is the easiest. Just cut to shape and cut the recesses in the sides. That might require a bit of tinkering, depending on which tools you have.
For fcs, you have to mold a new base and then cut the recesses. Not super difficult. Let me know if you want instructions.
MT bases are thicker, so you need to sand them down. But they are also less deep and less wide. I have remolded a few, but it requires some more work than remolding fcs. I prefer to let the sanded mt base "bottom" in the slot base mold and hence "steal" around a 1cm or the upper part of the fin (which gets molded into the new base.
For practical purposes, us base fins is by far the easiest. I hope we're gonna se fin manufacturers pre-cut the recesses into the us base fins soon. Then it will be super easy to convert.