I made a homemade screen for my large cooler/mashtun and it worked pretty well but not great. Would laying down a layer of cheese cloth under help or would i lose critical sugars by doing so?
What do you mean by not great? The grainbed should actually be doing all of the filtering. Anyway, to answer your question, you wouldn't loose any sugars, but you might get a stuck sparge.
I actually line my 5 gallon mashtun with a BIAB bag and have a copper pipe manifold I made underneath. It works great everything flows well and filters great. I usually aim for 75% efficiency but 4 of my last 5 brews have been a bit higher than that (yay bonus beer) . so I imagine you might have similar results. I was getting stuck mashes before the bag and not one since.