If it is just a plain loop, without a valve, then they are easy to make out of copper fittings from a plumbers merchant - that's what is on my exhaust loop.
I'm mystified by your post. The ad you've linked to clearly says 1 1/2" diameter. The post you replied to clearly said 25mm diameter. You even said "may be too big", when it's obvious it is too big. Why bother posting at all?
The original post wondered whether anyone knew where five 1" vented loops could be sourced more cheaply than the £25 or so most are listed at. There's no difficulty finding the things - lots of people make them - they're just expensive.
Well, I think that you will find that the Forespar ones are about as cheap as they get (well they certainly are in most parts of the world but could not comment for the UK) and that was the spirit (perhaps sadly now /forums/images/graemlins/crazy.gif) in which I offered the information.
One can find el cheapo non reinforced plastic ones intended for shore plumbing systems but I would not put such on a boat (apart from anything else the valves are usually problematical).
Didn't notice that he had asked about making them - actually I still haven't noticed him asking that /forums/images/graemlins/smile.gif.