You could use any material you wish! I hate spending time (and money) preparing custom molds for bodywork and I hate sanding, so I would go with flat panes cut in triangles and fitted on the inside of the tubes probably held in place with small screws or tie-wraps. CF panellin is what most teams prefer, but I have seen pretty unusual ideas that made me smile, from plexiglass (a bit heavy for my taste) and hemp, eco-friendly composites to corrugated plastic boards. The latter would cost close to nothing, you can find it easily, comes in a variety of colours and is sturdy enough and lightweight. I am pretty sure that there are companies that offer flame retardant compositions and are even able to print on it, so you do not need to apply sposor decals afterwards! I estimate that you can put it in place in a couple of hours and I am sure it would give you a point return in Cost Report as well!