2021 - What I did today

Spec it’s fastened to the frame with something similar to tent pegs, it seems to work.  I’ve propped it open with a couple of yoghurt pots but will alter that later.   
Edited to say I bought mine from Gumtree so i didn't know they were called Sun tunnels, it looks like they're no long for sale though.

I have just made up a box using pallet wood, now that you have posted the picture showing how you secure the covering I will make some alterations tomorrow, I have 3 of those larger tunnels and 1 smaller one, so will make up more boxes as they are ideal to use for hardening off some of my baskets and saves me having to lift them in and out the greenhouse, so many thanks Bren