From Scott - 2M0VSR:

https://jcoppens.com/ant/qfh/calc.en.php generates the paper template for the holes.

Quadrifilar helicoidal antenna - Javascript on-line calculator

Compared to solid core wire, it's quite easy to get it into shape, or close enough that it tunes almost smack centre in the band

I think the rods are M5 or 6 threaded rods

Whole thing can be built for less than £20

Think it was this that gave me the original idea: https://tf1ez.com/qfh-antenna-build/