Any chance of getting an empty snail shell and stuffing it with a bit of prawn/shrimp/cockle etc. Then get an empty pop/water bottle chop it into 3 sections, take out the middle section and try and join the two remaining sections together to form a smaller bottle. If you do it this way you'll still have a "bottom" to the bottle. Bury the bottle in the sand with just the "neck" and open top of the bottle sticking out. Chuck the snail in and keep your fingers crossed that the flatworm takes the bait.