I recently read an article in Fly Fishing Magazine about Hoppers. They went over many technics of fishing them and the different types but, the one thing they iterated through out the whole article was this. The foam hoppers were just too light when compared to a live hopper by 2-3 grains. They did not hit the water with the same fish attracting spat that the live hopper had. They best match to the live hopper was the Jay-Dave Hopper which has a weight almost identical. There is no reason I think not to use foam but remember you may have to put just the smallest amount of weight in order to get the splat need to attract the fish.