Polystyrene emergers can be effective patterns when smaller mayflies are hatching.
Just cut a piece of polystyrene suitable for the hook size you're using and roll it into a ball.
Then you wrap the ball tight in piece of ladies sock and tie it on the hook.
I like to use cdc for the legs since they move nicely and make the illusion that there's something alive at the surface.
I use curve emerger hooks, so the body is just bellow the surface film and the fly gets in that hanging emerger position.
Polystyrene balls floats very well and are quite visible.
These are not the most durable flies you'll see, but when you sight fishing and choose the fish you wanna catch, the "math" changes a little bit.
One fly per couple of nice graylings or browns, that sounds like a fair trade for me.