I use a round punchhole sieve on the top only, but my rows are white with snails. You will get less loss if you use on the bottom only. Remember all the snails are goning through repeats and things soon get blocked. Vermeeren Bros at keith make a self cleaning sieve for most makes of headers.
Id be trying to crushas many of them as possible before they get to to the sieve.
self cleaning punch hole sieves are quite easy to make by adding a number of 5-6 inch pieces if chain with a piece of flat steel welded onto them. works a treat for harvesting peas with bad snails.
like malleeboy said check your grain elevators top and bottom every time you unload as they might need regular cleaning. After learing the hard way with breakdowns from build up you get super paraniod about checking and removing build up!!