Cold Drawn Seamless Tube is what you need

Wall thickness and tube diameter are personal choice tbh and what you want from the cage (structural? just protection?). I have a main hoop piece in the garage I might be able to measure up soon if your really stuck.

Failing that ring up a custom cage builder, tell them your requirements and ask them what they recommend for your needs, sneak in the question about diamers and wall thicknesses.

Cheers
Dan