Anyone built their own Polytunnel from scratch?

Building a huge, cheap polytunnel with scaffold poles and water pipes - YouTube

 

We got a second-hand one here (they're ten-a-penny) so no tips for constructing one, but think carefully about how to ventilate it. The standard advice is to bury the edges of the plastic cover in slit trenches along the sides. The disadvantage with this is that, even if the ends are left open, it gets ridiculously hot in summer. The other approach is to use ropes criss-crossed over the cover so that you can raise the sides when necessary. If I was to do one again, I'd use a shade netting cover with a removable plastic winter cover to go on top .

 

Probably the same happened to it as the video you posted, failed.
Polytunnels flex in the wind, so I am not sure how you could support the sag, any upright supports would just snap or come loose when its windy.
I like the way how this guy built a pvc tunnel, more work involved in building it though. But again, will it withstand gales and heavy snow fall or probably collapse. It really needs braces going across the hoop, which should help to keep the shape of the tunnel.

PVC greenhouse in a day DIY - YouTube

What you would need is a brace across the hoop (width) something like the image below. This would help prevent the hoop sagging, you use clamps and then bolt a brace to help keep the hoop rigid. I know this is more for a metal hooped tunnel, but you could do it on a pvc hoop tunnel. Most polytunnel tunnel stockists sell the clips and braces, you just need something rigid but not to heavy.

 

There are quite a few threads on here about building polytunnels: that may be of interest.
See Steve's Polytunnels
and A few polytunnel work in progress pics

 

Thats a large size!! Can you get permission to erect that on an allotment?

Well it all depends on your budget, and various skills and crafts but, I would look for a used/pre-owned commercial model, and buy a cordless grinder to dismantle/fettle. Then look to investing in Vizqueen uv stabilised cover to suit.
*Just found this:
Polytunnel 18ft wide 66 ft long | in Taunton, Somerset | Gumtree (Somerset)
This is what you are working against:
www.highlandpolytunnels.co.uk/18x66-commercial-polytunnel-1083-p.asp

 

The only concern with using MDPE pipe, it is not UV resistant due to it being mainly used underground. Would it not of been better using black HDPE pipe instead, not only is it UV resistant, but it is more sturdy.

How to make your own polytunnel | Gardening & Permaculture Design (wordpress.com)

 

When we moved here 7 years ago I insisted we bought a polytunnel and found one online - only 4 x 8 metres so smaller than yours but it has scaffold type supports and a door at each end. We also bought the green netting cover which goes on from mid May to September for shade. That works well and we leave both doors open all summer.

In order to stop sagging there is a system of plastic covered wires stretched between the curved poles that form the roof - very handy for knotting support strings for tomatoes before I bought the stainless steel twirly posts.

It's unheated so I bought some metal IKEA shelving (Hyllis) with plastic covers which I use to keep seed trays warmer during the cool spring nights. They also keep the chooks off my babies when they go in there on rainy days.