I tried something like that years ago from a recipe I read in a book somewhere. Best I can remember, it floated the flies well, but there would be an oil slick around them on the water.look up the floatant that uses white gas and paraffin wax. I made a batch a few years ago and it's the best stuff I have ever used. The gas evaporates and leaves the wax and it's amazing how well they float.
I would share the recipe but I haven't made it in so long I can't remember it.