I JUST posted about this on a Festival thread!

I got this one on amazing and it's really big and the straps are padded so it won't get weird/sticky if you're wearing short sleeves during the day: https://www.amazon.com/gp/product/B074SZ2WD2/ref=oh_aui_detailpage_o00_s01?ie=UTF8&psc=1

Other people posted some suggestions as well:

https://www.reddit.com/r/festivals/comments/924pjr/clear_backpacks_unavoidable_so_whats_the_best_of/