Polo Rules

Don't forget corners are more dangerous than curves. Admittedly if you are hitting solid steel it's not going to make much difference, but i'd rather fall on a round pole than a square one.

+1 I wouldn't wanna see the Result of a collision with the joint between uprights and crossbar. How about 50mm solvent weld waste pipe. It's light, cheap, easy to work and readily available from any plumbers merchant. With a bit of thought a hacksaw and the right glue anyone could build goals. To collapse it for transpiration just don't glue all the fittings Drill through fitting and pipe and use pins to secure.