The corrugated plastic you used is available in several thicker sizes than the common 1/8". In my sign business I have used 4'x8' 1/2" corrugated plastic for campaign signs that have held up very well, but I think they were around $80 a sheet.
Here's some in 3/8"
http://corrugatedplastics.net/10mmCo...ticSheets.html
Phillip McGregor (OFC)
"I am neither a fire arms nor a ballistics expert, but I was a combat infantry officer in the Great War, and I absolutely know that the bullet from an infantry rifle has to be able to shoot through things." General Douglas MacArthur