I find most 12V stuff like modems and routers run off a 2.1mm or 2.5mm barrel jack. (And of course it doesn’t imply 12V). You’d need to bodge a double ended connector.

Maybe screw terminals would be ideal so you can cut your wall wart cable and wire it up. Most devices will have a bridge diode inside (total waste of electricity, but not going there right now).

I guess a cigarette lighter port could make sense but now we might be asking too much on the BoM.