I thought twin wall was in sheets rather than flexible
It's often more flexible than you want it to be - even the triple stuff can be warped out of its fixings by snow - on several occasions enough to leak, and occasionally enough to create a visible gap when all the movement happens at one side. (A good reason for minimising the expansion gap - a little too much expansion is much less troublesome). Yes, I know I should pin them centrally on each sheet - I do at the top, but it looks horrible at halfway, and risks leaks when done at the bottom.
Anyway, it can easily follow the curve of a 3m diameter tunnel if you lay it lengthways across the curve, and not so easily but do-able if the length of the sheet follows the curve (but only in warm weather - it's very, very tough but I wouldn't risk bending it "against the grain" below 15C).
Clamping it to pipes is probably more trouble than its worth - so it's best to design it from scratch (same applies to any other method in this thread). I would go for a frame-and-stringer approach like an upturned boat - it's easy to cut curved or elliptical frames from marine ply or even OSB.
Presumably both methods turn a polytunnel into a greenhouse, and building in roof vents becomes a breeze (sorry).
NB. You will probably need a few glazing bars when laying the sheets across the curve, but it's possible to cut into the cells both sides of a lengthways join so the cells interlock like hooked hands - losing a few cm of sheet in the join is a hell of a lot cheaper than an extra glazing bar, though the unclamped sheets will need extra stuff to hold them in place onto the frames (ropes or straps over the outside on every frame is the obvious choice, though metal strip or UPVC laths would look better).
The same kind of clamping could work for sheets along the curve too - but you would need dozens, scores of those pin-through fixings on the stringers - I'd probably want to put some stringers on the outside instead - U shaped aluminium would be good as it could create an array of gutters for harvesting rain - I wouldn't pay DIY prices for it, but I've already got a fair amount from skip-diving
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Cheers.