Update: first attempts to plan the scope around a section of 150mm pvc stormwater pipe have resulted in the realisation that there isn't going to be a nice-looking cheap way to make this scope. For one thing, stock PVC pipe isn't quite circular; it's also a bugger to work, resulting in all sorts of confetti, and worse still, it's very prone to static electicity effects...a dust-magnet substance.
So I'm on the web and looking for something a bit special.
CST Express seem to offer tube in the dimensions I need...but it seems pricey at $467 a metre.
Next line of inquiry is to a Chinese company called
Zibo Langda. I'll let you know what they say......