Yep, did this a few years back with an Igloo ice cold thingy. Grabbed a refill everytime we went past, still ice cold on the Sunday. Think the ambient temp in the lockup being lower than a small tent might have helped (better airflow?)
Basically we're going off 3 things.
1. They said on the BBC yesterday that they would announce their glastonbury schedule today and usually that tends to coincide with the full lineup announcement
2. It's come out on the 30th May the past 2 years, and in 2017
3. The Glade and Arcadia both said the full lineup and times will be out "end of May", which narrows it down to either today or tomorrow.
There is a small chance it might not be today and it might be tomorrow instead but the chances are much higher for today.
I'm gonna hazard a guess at 10am today