Really not much to it, figure out how may contacts you need to switch everything, buy a suitable relay, build it into an enclosure, and call it a day.
When I find a deal on a Dowkey, or similar TR switch, I toss it up on Ebay, get a ridiculous amount of money for it, buy and wire a relay, and pocket a nice profit.
Usually look for a surplus DPDT relay with 12Vdc, or 110Vac coil (depending on application), mount it into a small enclosure, with a couple auxilary connections from the other relay contacts, and suitable antenna connections.