I flashed routers with the Freifunk firmware. There is open firmware for existing routers. No need to solder.
Discussion
Never heard of Freifunk but I'll check it out
Probably not really relevant for your usecase. The argument was just that you can flash your router, which I did for Freifunk. I lived in an apartment building at the time and during summer, all the apartments around filled with tourists but didn't have Wifi, so I provided free wifi for up to 50 devices using Freifunk.