To be honest you could easily get some 8mm mild steel, weld up a box and make a robot that is spinner proof and cheap.

But it's not interesting, it's not remotely aggressive and just wouldn't be fun.

The real challenge is making something that's effective and on a budget.
It can be quite disheartening though when you see other machines that have hundreds of pounds spent on them, tools that you simply don't have and engineering skills you'd only obtain from working in the industry. But that isn't the only road to success.

Take Guilherme and his machine Galactus. Fastest spinning weapon at the champs this year. The brushless setup cost him ~£50 new. Design smart, buy smart.