I am making a Halloween cake and would like to create a spider web to place over tomb stones that I am standing up onto my cake. I thought that cotton candy would work, but, I didn't have much luck with it.

Any ideas would be greatly appreciated.

About the only thing I can think of is SugarVeil.

I would probably pipe very thin royal icing lines right on the stones.
It would be easier to make melted white candy melt webs on waxed paper, then just set against them...
If the stones are large enough you could use rice noodles...

I agree with7yyrt

Royal icing - you can pipe this onto clear plastic - draped over something rounder etc if need be to give it the stand away effect - and then when dry, just adhere it to the tomb stone with tiny dots of royal icing

or the chocolate method in the same way