I made a 2m. 4-element quad from 10mm copper pipe (about 3/8") and it was quite successful but far too heavy, dangerous to life and limb in fact. The modelling was done in both EZWpro and MMANA-GAL both of which tallied with each other just for once. I then did the same using stranded copper wire of 4mm x-sectional area, construction to be using 5mm diameter glass-fibre spacers on a 23mm diameter boom, the dimensions and characteristics were very similar.
The wire version performs like a piece of wet string, varying the element spacings makes little difference to the SWR so I presume the software-modelled dimensions are not correct and should differ more from the copper pipe version. On searching the internet I spotted that Cubex are making similar antennas and have spoken to a UK amateur who uses one and swears by it.
Anyone have any experience of this otherwise I may have to replicate the copper quad with an aluminium rod one? It's a pity because the wire one weighs very little and the glass-fibre is very cheap.