Multiple down votes for hardware cloth!
Have to switch to Plan B.
The material at the top of the spillway guard does not even need to be permeable. The purpose is to block off the floating debris from reaching the outlet pipe hood.
Would treated ground contact wood last longer? Do you have any plastic materials in the back of the shed that would work?
Another idea would be to make a floating square of pvc pipe (to work like the opposite of the feeding rings used to contain floating fish food). If built correctly, that should work to block off floating debris outside of the square, and create a raft of crap that you can later scoop out of the pond.
I am a little worried that a stiff wind would push some of the debris over the top of the rounded pipe. Maybe attach some weld wire to the pvc, so that it sticks up 4" above and down 4" below the pipe. If that worked, then you wouldn't even need to drive t-posts offshore. You could just attach your "guard" on the shore at two points.
You could also pull it out for winter or if it is too ugly for a swim party.
