Matt, I've been interested in the same topic. I got the hint that large RIB radar arches might come close, but those I have seen on the Ebay and the like are only about half-height and half-width in the 200-500 GBP range. I guess that beyond that, shipping becomes an issue that limits mass-market sales. One could ask an SS workshop to cut and extend, of course.
Another alternative could be low-cost DIY, have you seen
I still have the Aluminium one that was on my Moody 38cc if you are interested. I am only after £60 for it if it helps. This includes the stainless wire stays and bottle screws that tension it up, but not the panels.
It is not the most elegant but it does the job and is in good condition. I would like to get rid of it from my garden which is where it currently resides!
The only problem is we are in N Suffolk and it would really need to be collected as it is a bit awkward to carry other than on a roof rack.
Matt, took me 18 months to find mine on Wight Bay a small free adds site for those on the IOW. Your other options is to keep an eye on Ribnet as they come up regularly. However, at £60 I would take Chris up with his offer, even if it means cutting it down a bit.