I made my tractors out or 2"x2" i ripped down from a bunch of salvaged treated 2x4 i had. i used a tarp on mine as well i got 3 seasons work of use out of the tarps. But my biggest issue was the wind one spring my tractor actually picked up and flew across my yard and across the road, im just glad there was no birds in it at the time. How much does the PVC weight im just scared if i built one out of PVC it would be even lighter and blow around more.