Snails, man I hate them buggers Tom. Hardest thing to imitate I know of. Try Kaufmanns catalogue p.110. Not a very good pattern if you ask me.
Try Rainys float foam in various colors rolled up and impaled on a large dry fly hook. Trouble is the dang things come in so many different color variations. Try to match the naturals with panatone pens on white float foam. Many people think a gob of yellow powerbait simulates a snail!
Who cares?
There is only one way to correctly fish the chronomid. In front of feeding fish,hanging from a strike indicator,at the correct depth. There are days the fish like it moving,but these are seldom.
If they do just twitch it an inch or so every so often, or you can put on a small splitshot and drag the fly ultraslow under the indicator on non fly only lakes.
Depth is critical as is the correct area in the lake. Generally in spring it's somewhere shoreline oriented in 3' to 12' water. Use a fishnbuddy to find the fish or go where the swallows are feeding.
Sometimes on rare occasions you will need to fish Chronomid emergers or adults, but those occasions are rare also.