The thing with something plastic is you don't have to wash it at all, just mop up spills. The thing is, when us humans make a big deal about something they view it as attention, and to them attention is gooooood. Doesn't matter if it's good or bad, it's still attention. By having something plastic it means you're a bit more chilled out if it happens, it's just mop up with some tissue, wipe some vinegar, done. Whereas if it's something you have to remove, put in the wash, replace, it's a bigger deal. Hope that makes sense!
Also you didn't mention where you feed her. Again if it's associated with the bed in her mine she may think 'ah bed = food' and do things on the bed that get attention.
I had a lot of trouble with my bunnies and them weeing in the bed. I did various things. We built a ramp so Mischa could get down as he struggled with that, I also used the tarp as mentioned above. The other thing connected was that in the morning the first thing I'd do after getting out of bed was to feed them. So Mischa, bless him, in his little food orientated mind worked out that by peeing on me early in the morning I'd get up, feed them and then sort the bed out. By peeing on me he got food earlier! I started making sure that before I fed them I went out of the room for a few minutes and then back in so he started to associate me coming through the door with food, rather than me in bed.