There are quite a few demand-type feeders available on the market that are more economical than an automatic feeder. As I understand it, most use a rigid dangling rod that the fish bump into that releases the feed. I don't know why you couldn't manufacture one yourself if you felt so inclined. Simply drilling a hole in the bottom of a suspended 5-gallon bucket and suspending a rod from the lid through that hole might work. This is very similar in concept to the Kenco wind feeder for deer that retails for about $20.