I've used contact cement thinned with Coleman stove fuel (white gas). I don't remember the dilution ratio. The result is a very strong webbing. I should also mention that while rubber cement does make great webs, they are extremely flammable. They also dry up and crumble in a few days. Also, no matter what you use for webs, you can accent them using talcum powder-- remember, one reason you can see real webs is they have collected some dust.
Scott