The arms are 500mm lengths of 20mm x 6mm flat steel bar, I got them from a local steel works who cut them to length for me. the 6 bars cost £3 and is about the cheapest and simplest way to go.
the arms where marked out and drilled on the pillar drill then corners where ground and they where then painted with a rust killing metal primer.
Again I could have skipped the painting until after assembly but the better the job is done the longer it will last.
The picture shows a completed hub and the cross section of the other one.
I used 6mm cup square bolts and nylock nuts and washers to bolt this all together 110mm for the center of the hub and 50mm for the outer edge.