Yes I've had to solve this too - not exactly for your scenario though.
I'm afraid the right DIY way to go about this would be to design your own PCB and put it in a box.
It actually sounds like a nice beginner PCB project. Pick the connectors that suit you, pick some chips (there are many chip models available, this is no rocket science), throw them all on a PCB, take care to block the chips' respective power input with suitable MLCC's, select a power supply (a +5V wall wart will do). Once you get some idea how large the PCB would have to be, try to get a suitable box for the PCB. There are PCB prototyping services that are willing to make and ship a small PCB for a couple USD worldwide... Try Kicad or Eagle Free to design the PCB.
For RS232, take a look at MAX232 and its equivalents. If you don't really need to level-shift the signals, it would probably be possible to repeat RS232 directly at the desired levels using some op-amp or comparator, ugly as it may feel...
If PCB design is not what you meant by DIY, the answer would be a little difficult :-) A passive split should work for 3 "consumer" / "line receiver" devices, but the chances of mischief are greater as you add more devices.
Meinberg have some in-house boxes (cigarette box size) for this very purpose, as an accessory to their timing GPS receivers - but the price tag is not quite DIY level.
Some vendors of "industrial control gadgets" have RS232 isolators and RS232/485 converters. These could be interesting to you if your wiring distances are non-trivial or if your local grounding is not perfectly even.