^^^ use the BEC voltage and a relay that can handle the current. The switch energizes the relay which then allows the current to pass from your 24v source to where it's going. The 24v source could be used to energize the relay and the switch powered by the rx would turn that on/off. That much continuous current through anything other than 8 AWG is asking for trouble. Whatever path you choose make sure that it can handle that kind of power.