Here is the Wondom BCPB2 3S 18650 MPPT BMS I am talking about. Leaving out the DC connector, it is still 31.5 mm high with the electrolytic capacitors being the next highest components. However, if necessary, 1 mm can still be shaved off by replacing the 18650 cell holders.

It is pretty cheap, to the point that you perhaps could integrate it with your transceiver by default. The MPPT allows it to be used with a 18 to 24 V solar panel.

I got the idea of a video from Gil, F4WBY. He also employs this to supply power to his QCX radios. (Do not follow his suggestion of soldering directly to the cell terminals, though. Just apply once external power to the board to get it running.)

The newer version PCBs have extra headers to drive external power level LEDs, without the need to add any extra series resistors.