I didn't make it to the festival this year (instead I sat watching it at home with my 7 week old son). However in years gone past, this has been a game changer when we've stayed in the scout tents at WV
https://www.decathlon.co.uk/p/camping-tarp-tarp-fresh/_/R-p-312174?mc=8572388&c=WHITE
It is the perfect size to throw over the top of them and ties down on either side. It is not as good as a blackout tent, but it'll buy you a few extra hours in the morning, once the sun comes up (and it makes your tent really easy to spot).