I use these guys at work for mast rigging stuff, also used them when I wanted some HT HDG ones for my boat trailer. https://www.flacc.co.uk/telecom-products/fasteners/square-bolts/
I'd be happiest if those pieces were formed while red hot (at least) to ensure the grain follows the form and minimises risk of overtraining and potential crack formation.
if they wernt available at a reasonable price easy enough to make a simple jig . get some high tensile rod and heat them with oxyacetylene on the bends making a tight curve and then thread them . just put these in another post I knocked up for someone