We have found a lot less debris on the floor of the indoor range while sweeping up as well, because the plywood tended to splinter when the bullets hit it.We go through a sheet per month, and often if the local hardware store has a broken sheet they throw it in. Saves them from scrapping it and actually getting some use out of it.
We were offered a bunch of "pressed paper'' appx 1/16" thick but it fell apart in the rain and local kids loved it as well for whatever projects they had in mind.
The corrugated plastic is nice as it doesn't bend tacks and staples are easily removed.
Cat