How can I keep my guinea pigs from pooping in their bed?

I have a fleece bed called like the critter cup or something that my guinea pigs both sleep in. I got them yesterday off craigslist and they're about 1 year old. I have an area with bedding and a litter box, but they don't use it. I put hamster litter in (I couldnt find guinea pig litter) and some of their poop in it so they'd get the idea. I don't want to wash the bed in the washing machine every day. So how can i litterbox train them?