You can rig something yourself with a toilet float mechanism. It would be a LOT more compact, and probably not very much more expensive, to just buy the purpose-made thing and be done with it.
Be aware, though, that a) it won't work reliably during the wintertime, because the feed line will freeze up and even if you have heat tape on the feed line (which is IMO pointlessly risky considering the frequency with which heat tapes cause fires) you still have a good chance of a thin skim of ice forming around the shutoff mechanism and locking things up. It is *possible* to construct relatively frostproof auto waterer systems, for large livestock anyhow, but it is never *cheap*.
Also, I don't know whether you've used livestock auto waterer systems before but you need to realize they will ALL overflow sometimes. Really truly. The better ones will only overflow occasionally; others may flood the coop with depressing regularity. At the very least, you want it located somewhere that offers floodwater an easy 'out' rather than having it soak all your bedding and get your coop structure so wet it starts to rot. 
Good luck,
Pat