I bet this would work for a corn fly. Get some glo bug yarn which us steelheaders use for tying egg flies. It comes in all types of colors.
Tie them up on small strong hooks, they are really simple and will sink very slowly. Probably would need to use a couple of very small split shot in front of it, or a slow sink tip head to get it to sink to bottom.
Actually I have a bunch of yellow yarn I hardly ever use if you want give me your address and I will ship some out to you. Send you a few yarn flies to see what I mean also.