Well, I used to have dust covers that I made out of large clear plastic bags but over the years I just stopped worrying about it. Close to 3 decades ago I was playing at an outdoor festival north of Orillia. Back then I used to be quite the bad party animal. I had this covers on my 'boards since they were set up onstage and I wasn't playing them. Of course they were plugged in but not turned on. We had already played our set and since I was expecting to play again the next day I just left them there while I got, um, let's just say well into my cups. And mushrooms, and probably other things too. At some time I managed to get into my tent and slept through a monsoon. I woke up because, A, I really needed to pee and B, the tent had collapsed on top of me and whoever I was sharing the tent with. Got up, noticed that it had rained a bit totally unaware there had been a huge storm. When I toddled over to the stage I noticed that the plastic cover was long gone and there were puddles on my keyboard. I unplugged the power cord and gently bumped it. Water sloshed out from under the keys. It was full to the top. I said some words. Probably ones that started with an "f" and tipped it over to let at least 2 gallons out. Fortunately that was the only piece of gear that got drenched. My other keyboard was under cover somewhere. Maybe I only had the one. Can't say. My keyboard amp was fine. I eventually got it home, penned it up to let it dry. After a week I turned it on and it worked. It took it a while to figure out how but only about 10 minutes.
On the positive side, my ex wife and her husband were there. PS I didn't like her then. She and he were having a threesome with another girl when the husband said something stupid which got the ex furious. She ran out of their tent naked with him in chase. In the dark, she tripped over a tent rope or peg, fell and he landed on her and snapped her leg. When I heard about this, I completely forgot about my sad Roland U20 keyboard and laughed and laughed and laughed! And then got drunk! I wasn't driving so it was good!