Utterly Mad

Cardboard is your best option, corrugated cardboard at that, similar concept to angled armor and it'll also be a little stronger structural wise.
Aluminium frame is also likely to be your best option to allow you to use some kind of foot pedal for propulsion, which will cut costs since you won't have to build an engine.

Although a fixed cannon will result in a much simpler build than one with a turret and may be a better choice for your first build, refine and improve it later, you could end up adding a turret onto it with it's fixed turret at a later date.