The best snail trap in my opinion: cucumber.
The housing only prevents larger animals from reaching the cucumber (especially shrimps ;-) ). I clean the trap every morning, removing dozens of snails. 
The cucumber should either be bio or the peel should be cleaned up or removed.
You can attach an 8 mm aluminium rod to the housing.

Just a tip: With this trap you will get rid of 95% of the plague (e.g. Physa sp.) in a short time (1-2 weeks). But never to 100%. If you use snakes of anentome elena, then you will also kill the remaining 5%. Just note that anentome elena will die out afterwards when there are no more snails to eat, so you'd better give them up in time.