Closed-cell foam, 2mm (back strip) Brown thin foam (1 strip)
Thin leather, foam, or rubber (tail) Brown silly legs nymph size (1 strip)
How to tie it
Lay a thread base along the shank and tie in a thin leather or foam tail at the bend, about one shank length long.
Bind a strip of brown foam at the bend on top, pointing back over the tail, to be pulled forward later as the back.
Spin or stack a bunch of deer belly hair just ahead of the tie-in, let it flare, and pack it back tight with a hair packer.
Add and pack successive bunches of flared deer hair forward, keeping the packing firm, until you reach the 85% mark; leave room at the eye.
Whip finish, remove the fly, and trim the spun hair flat on the bottom and rounded on top into a slim mouse body (take less than you think, so the fly stays light and floats).
Pull the brown foam strip forward over the trimmed top as a smooth back and re-attach thread at the eye to bind it down.
Build a small thread head, whip finish, and cement; trim the foam nose short and square.