Prescott Plastics 3/4" - 0.75" Square Plastic Hole Plugs, Inserts, Black End Caps for Metal Tubing, Fences, Glide Protection from Chair Legs and Furniture (50 Pack) - Walmart.com
2-1/4" OD x 2" ID Clear Extruded Square Acrylic Tubing | U.S. Plastic Corp.
Plastic square tube 50mm x 50mm x 3mm white
Amazon.com: Acrylic Rigid Square Tube, Clear, 1/4" ID x 3/8" OD x 36" L - Pack of 4 : Industrial & Scientific
PVC GRAY SQUARE TUBE Plastic Sheet - Mobile
Clear Extruded Acrylic Square Tubing (1/16 : TAP Plastics