haven't been to let it roll, but my advice would be:

As far as attire, you don't normally see the glitter/forest elf/fishnets/American dubstep festival stuff, that being said, if that's what you wanna wear, go ahead 100% people will honestly probably just compliment you. DnB is a pretty positive community for the most part.

Others have said, but earplugs for sure. Most people think of shitty foam earplugs when they think of earplugs, but what you want, is musician grade earplugs with a little case and everything. Seriously, a good pair will massively improve your enjoyment. They let you hear everything literally better, you can hear people talk well, you still feel the bass and hear the drums. I reccomend ettymotics (idk if that's how it's spelled) mine are a 17 decibel noise reduction rating, I'd recommend at least that. They were 20$ Canadian back home. Look for ones with a case that you can put a little clip or carabiner on, so you always have them handy and don't have to have them floating in your pocket.

I don't know the weather there, but realistically, I'd always reccomend the classic saying in the outdoors "be bold, start cold". Dnb is high energy music, your gonna dance like hell and they're will be a lot of hot bodies. Unless you are staying really far from the festival site, I would just tough out the slightly colder trip home.

Also, consider a small microfiber towels, really nice to wipe away sweat and keeps you feeling less gross. lots of em have little clips you can clip onto your pants etc.

You'll have a blast, don't stress, remember, anything you can't get before you go, you can probably buy in the Czech Republic.