I've taken this idea from some of the other contributors on this site. The very last photo shows my first attempt. I found this to be ok but not very effective. My method has less preparation and assembly work involved. It stops pets from drinking the beer and keeps out the rain. Materials and tools - empty soda bottles or plastic milk cartons and a marker if you want to be extra neat when making your cuts.
I made four traps and placed them in my garden. The next morning I had over sixty slugs and a dozen or so snails. I strained all the bottles, dumped the dead slugs and snails where the birds could get them and then reused the beer and the bottles. I get about three or four days use from one can of beer. One 500ml can will do three bottles.
Cut a window in the bottle. Only make the cuts on three sides, leaving a flap of plastic still attached to the bottle.