You could install powerpole connectors on all the bits, including on each side of the diodes. You can leave out the switch then. It's all a bit risky though, as you know, the voltage LIPOs are too high for almost any radio. Perhaps include a voltage alarm that is enabled whether the radio is on or not. Odd things happen on summit especially in inclement weather.
You can also reduce the voltage using a FET or a PNP transistor as a series regulator. The 3v lost in the regulator x current drawn is then the lost power. The dissipation in the diode or the pass transistor is lost power.
I have a similar 4S LIPO and plan to use an inverter such as Peter mentioned, but with an overvoltage crowbar circuit as suggested in another forum by Ron vk3afw.
Running the radio from 13v instead of 16.8 should extend the battery life for each charge, as the power used from the battery will be 13/16.8 of what it would be when dropping the voltage via diodes or series pass device. Inverters aren't lossless though, I will acknowledge that before it is pointed out. But they are likely to be over 90% efficiency and can be left running all the time whatever the charge level is in the battery (this has the potential to over-discharge the battery, of course. no free lunches...)
hope these thoughts are useful.
73 Andrew vk1da/vk2uh