A magazine extension for a model 12 is not going to be a simple thing. One thing that comes to mind is to have a long magazine made using 2 tubes for a model 12, cutting, welding and such. Then, changing the magazine tube from the short one to the long one is going to be a pain. The entire front end has to be disassembled in order to change the tube. Your mileage may vary!!
Blackfoot