First, I used combinations of switches to select the mode of my APM. But IMHO, there is hardly anything more counter-intuitive and if things hit the fan, you can be sure to switch to the wrong mode out of pure hectic.
Next, I replaced a pot of my transmitter with a 6-position switch and variable resistors. That is already better, but to get to one mode, you likely have to switch through other modes. Also, without any markings, again, not very intuitive and it's impossible to see at one glance which mode you are in.
Finally, I hacked together a small board with 6 pushbuttons, 6 LEDs and an Arduino Pro Mini. The Arduino creates an 8-channel PPM stream of which channel 1 contains the data for the mode. The whole thing connects to the trainer port of my 9x. Additionally, I steal 5V from the display output of my FrSky module but theoretically, the Arduino could also be powered by the 3S transmitter LiPo. The program is pretty easy. Check the buttons and set the respective PPM value. The LEDs are not set by the buttons but by the actual value of PPM1 - just to be safe, in case e.g. the Arduino resets or something.
Here's the schematic: