The main reason you can't have a full roll cage in most cars without having it engineered, is mainly due to the cage impeading on getting in out of the car.
So being that a full metal cage would impead you getting in, so will a PVC pipe cage.
IF you only wanted it for displays you could have your rear cage in perminantly and then "bolt" in your front supports when car is being displayed.

You would probably need to do this anyway as PVC pipes will more then likley break as you climb over it to get in the car.

why not get some toilet paper harnesses aswell...


won't save you, but it'd look smashing and finish off the sewerage theme, too!

The reason that you can't have a full cage (or rear passengers with a half cage) is not because of impeding access, it is because you can hit your head on them in an accident.

A thin steel fake cage will still hurt you if you bash your head on it.
A PVC fake cage could splinter in a crash.

I would personally go with the cardboard fake cage.