I modified this Acoms 4ch FM TX about 12 yrs ago to use one of the stick channels for a rotary switch on a 3ch aircraft,- with flaps for the 4th channel. I managed to fit a pot (8k from memory - matches existing pots) into the case and took the wires from the pot supply from the board, cut them and fitted 3 pin M+F plugs (from an old printer) to both sides, plus another plug for the new pot. I can plug in the rotary or stick pots to suit different models if needed (ie std 4ch or 3 stick and 1 rotary ch.) This can probably be done with many simple TXs where a rotary is needed for fine control (maybe a fire monitor or similar.) or you could make them all rotary for a special purpose. I also fitted a toggle on/off sw as the slide sw was defective
JB