It's weird. Stillwaters the world over have snails in them and fish, not only trout, like eating them. They're slow moving, full of protein and minerals. They're probably tasty and abundant, especially in reservoirs where they can graze on the algaes that grow on the dam wall. What's not to like? But for some reason very few people fish them. OK they're not an everyday "hatch", but lots of common immitations that people carry only get occasional use. Corixa, hoglouse or daddies aren't everyday flies either, but no-one looks at you askance when you suggest trying one of them. I really don't understand the resistance, anytime I've experienced snail feeding I've really enjoyed it. Sometimes it's easy to work out that theyre on snails because you see them floating just subsurface or even crawling on your boat. Other times it's not so easy, and fishing can be tough until you manage to pick up a fish, and it's so full of snails that feels like a bag of marbles. One of the dams where I grew up was particularly snaily, especially in late summer and I was always hoping they'd be on whenever I was up there.
One issue might be that anytime you do see snail imitations in a magazine or some forum, they're almost always daft cartoonish looking things. I'm sure you've seen the type, spiralled foam shells coated in resin and maybe even antenna. A pain to tie and they don't float the right way up. It's hard to take them seriously. If you're going to fish snails you only need 2 patterns, a floating snail and a sinking snail neither of them are difficult or time consuming. The first floating snail I ever used came from a book I found in my local library, basically it was a lump of cork, glued to a hook with peacock herl wrapped around it. It caught plenty of fish but took ages to tie and wasn't very durable. Then about 25 years ago I discovered Peter Dobbs's Bigfoot (pictured), ready in 2 minutes, easier to see and absolutely deadly on its day. I don't think there is a better floating snail pattern. Your sinking fly could be a chunky black & peacock spider, but I think a coch & peacock is probably a bit better. For either pattern you might want to tweak the size and colour a bit but the basic profile doesn't need to change. So 2 flies with 3 materials between them, you could put more than a season's worth in your box in under half an hour. And they work! They're so easy to fish too, do nothing, impart no action, just let the flies drift. Takes are almost always long deliberate draws, much like straight lining buzzers.
If you've never tried them, have a look in the margins of your local water when you find some snails, tie appropriately sized imitations and give them a try. You'll not be disappointed.