Mossberg slug barrels are easy to find and as far as I can determine, the 20ga fully rifled barrels have a 1 in 36” twist.
Properly modified one should work for a monoblock or shoe lump barrel.
I’ve entertained this very idea using a single barrel trap gun I have.
It seems like the most straightforward conversion for an amateur gunsmith. Of course, I could just shorten the barrel and see how she does as a 12 smoothbore. Maybe some day…