Mount the 3x long hook, tie in a long forked moose tail. A shank length and a half; Hex are enormous.
Dub a big, robust yellow body with a real taper, to the 60% mark. This is the one mayfly where more dubbing is correct.
Stack and tie in a large upright elk wing.
Wrap mixed grizzly and brown hackle, heavy. This fly must float a size 4 body in the dark.
Whip finish, cement well.
Substitutions
Body instead of Yellow dubbing: Pale yellow Superfine — Easier to dub the big body; Yellow foam underbody — Keeps this big fly floating; Yellow yarn — Quick body for tying multiples
Tail instead of Moose body hair: Brown bucktail — Long fibers for this big fly; Micro Fibbets — Cleaner split
Wing instead of Elk hair: Coastal deer hair — More hollow, bigger profile; White calf tail — Visible in the dark when Hex hatch