Rig a shank with a trailing hook loop of braid or wire, or use a stinger-hook shank system.
At the rear station, build the first "bump": a ball of dubbing, then a sparse ring of ostrich or rhea fibers flaring around it, plus flash. The bump props the fibers out so they breathe.
Wrap the dubbing brush body forward over the middle of the shank. Keep it slim; the profile comes from the stations, not the body.
At the front station, build the second, larger bump: dubbing ball, ostrich or rhea flared around it, schlappen or marabou collar over that, flash throughout.
Tie in dumbbell or bead chain eyes below the front station.
Whip finish. The finished fly is mostly water and movement, two flowing fiber stations on a nearly bare shank.
Substitutions
Hackle instead of Ostrich or rhea: Rhea — Longer fibers, classic choice for big water; Ostrich herl — Shorter, more available; Schlappen — Longer fibers, different movement; Marabou — Softest option, maximum movement
Body instead of Dubbing brush: Ice Dub brush (custom) — Make with dubbing loop; EP Foxy Brush — Pre-made, saves time; Laser Dub brush — Flashy, spikey profile; Crosscut rabbit — Maximum movement