try using the concept of the commercial fishermen - funnel entrance on a two liter bottle. Cut the top off the 2 liter, invert the top into the bottom piece, secure with zap strap. weigh the bottle down with rocks/gravel, put food inside the bottle. lay it on its side. Shrimps should be big enough to get into the hole but most fish won't however hermits and crabs can get in too.
try using those formula 2 marine pellets instead. they tend to hold and give off scent longer than bloodworms. mysis is good for giving off scent as well because of their fatty content.
give that a try and let us know how it works out.
