I can't believe you did that, filter or no filter, one grain in the workings of the lens and you'll be having a wallet attack.
I carry a small groundsheet, it's about 3.5x5 foot (ish) reflective on one side, tough plastic or vinal stuff, weighs nothing and not only is it something to put your kit on but I've laid on it to shoot low angle stuff, keeps me clean-ish, it's also a LOT warmer than the cold ground if I have to spend a night out or wait for something to start.
I spent two and a half hours sitting on cold cement the other day, quite comfortable sitting on the ground sheet (waiting for something)