Being a plumber for over 50 yrs , I haven`t seen very many pvc pipes that were perfectly straight , or didn`t take slightly bend/set after removing from a shipping bundle . Cant even lay them on the ground w/o some distortion , flexable in anything under 3'' , or schedule 80 too , even schedule 80 wont stay straight ...
think I`d have more faith in angle iron .
I agree there is flexibility in the pvc pipes in a free state. When on the level floor in my garage I can align the ends of the matched pairs and they are an equal length. And are straight to the floor.
Look close at the photo and you will see an adjustable threaded support leg just forward of the coupler. That is needed to eliminate down flex.
I checked the up and down level of the pvc frame and it is level. I also used the same 4 foot level to look horizontal bends, there are none.
I also used a 3 foot carpenters square to verify the corners are at 90 degrees, they are.
The seven foot sections at the ends of the car are very rigid. The long sections on the sides did require a vertical support.