Like this guy says, finding a mesh fine enough to stop a £1 coin yet without impairing visibility may well be impossible.

Stadium designer Paul Fletcher feels the idea is unworkable as finding a mesh netting small enough to stop coins could prove impossible.
Fletcher worked on the design of Wembley Stadium, as well as stadiums at Bolton, Coventry and Huddersfield and was formerly chief executive of Burnley.
He said: "To stop a £1 coin going onto the pitch is going to take some fine mesh netting and I have never seen that.
"In foreign stadiums they do have netting, but that is to stop bigger objects like flares. I have certainly never looked at finding something to stop coins when designing stadia.
"I have never seen a mesh you can see through without feeling it affects your view of the game."