I don't know if they still make it, but Braun (I'm pretty sure that's who it was) made a little 1-inch cube that you slipped onto the hotfoot of any strobe, turning it into a photo-actived slave. It worked great and with two flashes (the main and a slave), I could light up an entire reception hall. The thing was so cheap that it would have been foolish to build anything from scratch -- and anything I could have built would have had a volume five times bigger than that little cube. Alas, but years ago, that little accessory was stolen from my home .... along with my Nikon F2, three lenses, strobe, extension tubes, etc. Man, do I ever miss that camera.
Dean