The basic plan is to use junkbox small signal silicon NPN transistors to implement level shifting from RS-232 to 5V TTL compatible. These may be of the type you buy by the bagful, if you pay more than 10 cents a pop for them, they're the wrong ones.
The convertors will consist of two +/- 12-30V down to 5V level converters and two 5V logic level to +/- 12V or better RS-232 levels convertors. This will allow TX, RX, CTS, RTS lines to be implemented, which with signal ground is a 5 wire interface. Should additional signals be required, these might be able to be faked, i.e. tied high or low, or additional circuits can be added as required. The down convertor will require a logic inversion, or NOT operation on it's output as the sense of RS-232 is reversed compared to TTL. This will require a further transistor per output, or parts of a 74 series or other standard logic part.
Now you will anticipate the problem that we need multiple voltages. I want to build this to run off a USB battery pack or adapter, for post-apocalyptic authenticity we will assume this is my solar powered 1800mAh battery, that manages 500mA output. If however you are anticipating running it off 12 or 24V solar in your personal apocalypse, you may find you will not need the first half of the voltage conversion circuit.
I will be seriously abusing the design of the buck boost converter shown on that adafruit link. For a start, FETs, pffft, where we're going we don't need FETs... well actually they'd be convenient but the junk and spares boxes didn't yield any so I'm gonna disgrace the crap out of it by making do with a darlington pair. Yeah, they suck at high frequencies, so lets not do high frequencies, and if we don't go over about 20khz, we don't need a schottky diode either, regular boring old slow rectifier should do the trick. If however, you wanna go 12V supply, you can miss this and nail a 7805 into board to get yer 5V or do what you want for that.
Oscillator, to provide the bucks to make the boost... now here we can get really creative. We just need them wiggles. Several options, nand gate oscillator, nor gate oscillator, hex inverter oscillator, astable multivibrator in transistor or IC, or a 555 timer. I happen to have a 555 timer, and unless I decide I'd rather use an inverting buffer or a nand package to NOT the RS-232 sense I might use one of those.
Then also a flipper of the voltage, negative volts from pos to make ze schplitten railen.
So as we see this is a just a whole soup of different circuit elements, where we can switch beef out for chicken, or onions out for leeks and still get some kind of soup at the end.