Acrylic

• Black, Ebay, $5/sf, 1/8" - [1]
• Menards 3 sf, 0.08 - $3/sf - [2]
• 6 Square feet - $6/sf + shipping - [3]
• $23/sq ft, 1/4" - [4]
• $9/sf - 1/4" - https://www.mcmaster.com/#acrylic/=1653xn3
• 2x4 - 1/8" - $37 - [5]
• $6/sf - 1/4" - https://www.menards.com/main/building-materials/acrylic-glass-sheeting/clear-colorless-acrylic-sheets/p-1444441881915.htm
• $4/sf - 0.1" - Sutherlands - [6]

Notes

• You can get 1/32" polycarbonate, 4x8 sheets, at about $70. This is about $2/sf. Would need appropriate structural support, but otherwise it's pretty much indestructible.