The 20 incher is a great stonefly imitation. This version is great if you need a heavy fly to get you down as deep as possible. If you don't want your 20 incher to be as heavy as the one Mike ties, skip the weighted wire.
How to Tie a Slightly Modified 20 Incher from Chris Eaton on Vimeo.
The Ingredients you need:
hook: sz 6-10, nymph style
tail: Goose Biot, brown
rib: Pearl Flashabou and copper wire
abdomen: Peacock Herl (5-6)
wing case: Swiss Straw
legs: Hungarian Partridge
thorax: gray dubbing (gold if you want a golden stone)
weight: .025 Lead or non lead wire
bead: 3/16in tungsten bead