I stumbled across these the other day, www.camocaps.net/. I am pretty seasoned at sneaking shit into the gorge but I think I might have to give these a try this year.
You don't need to purchase these.
To remove the plastic cap w/o breaking the tamper-resistant safety ring seal from any plastic bottle- • heat one cup of water til it boils • hold bottle upside down and insert bottle's top approx. 1" into hot water for 45 seconds. • remove bottle and immediately and slowly unscrew bottle cap.
We did this last year with a case of water bottles and brought about 20 successfully removed caps and empty bottles to the festival.
Which brand of water did you use? I tried this on 5 bottles of some random brand I found at work and I couldn't get any of them to come off.