Some buns really do like laying on the cool plastic. Leroy doesn't seem to care either way, so sometimes I use a towel, when I have a clean rag, and sometimes I leave it out.
I've found that cleaning is a LOT easier when the bottom is bare (just scoop up any loose bits of hay and take a couple paper towels and some 50/50 water/vinegar solution to it and you're done) or with a towel (just wrap up the towel around the loose bits of hay/poops, shake it outside and toss it in the wash and then do the same with the vingear/water). Also, limiting the bedding/litter to just the litter box will her her with litter box training. Sometimes having litter everywhere can confuse them. How old is Loki? Has she been spayed yet? (sometimes younger buns or intact buns are more difficult to litter train). If you find that you are having trouble litter training, and you don't aready put hay in the litter box, you should try that since buns like to eat and poop simultaneously, it will encourage her to use the box.
One thing with the towel, some buns like digging/eating towels and will pull apart the little strings. If they swallow those or get one wrapped around their teeth, they could be in trouble. If Loki likes chewing facbric, try a cheep fleece blanket (fleece doesn't have strings like a regular towel) or go to the fabric store and see if you can get some scraps of fleece for a few bucks.
Also, depending on how big Loki is or if she's still growing, you may find that the corner liter box isnt big enough. Theyre great for saving space, but the space that is saved isn't always worth it. I used to have one of those for Leroy and it did alright, he just about took up all the space in it, but it worked fine. About a month or so ago, he started scooting his bum wayyyyy up to tthe edge and since the front edge is faily low, he'd pee and poop stright over the side and make huge messes all over the place. I often thought he did it on purpose. I used to try to push him forward when I saw him with his butt over the edge, but as soon as he saw me coming, he'd stare straight at me and pee, and then run off. I have a large cat litter box in my living room for when he's out playing and I've never had a problem with him peeing over the edge in that one, one becasue it's bigger and two becasue the sides are higer, so I got another one of those for his cage (about 3 or 4 bucks at Walmart) and it's been fine now. One thing about litter boxes, some buns like to hang out in them, so if Loki is one of those bunnies, it would be good to get her a larger box so she isn't laying directly in her poop.