Your current set up is almost perfect, actually. You would only need one of those bins to drain, right? Why not drill a 1" hole in bin 3, find a rubber fitting you can seal and tighten, and then run a length of 1/2" vinyl tubing out of it into your sink drain? Run another tube from the faucet into the bin and you've got a wash sink. When you're done, stack 'em back up in the corner. No need for the developer and stop bins to drain.