I've just made myself a fleece liner to try out and have used a cotton mattress protector as the inner layer. So far it seems to be working really well (despite my incredibly wonky sewing skills) and, though I've put newspaper underneath, I don't seem to need it. The mattress protector seems to be a doing a good job. Both the top and bottom layers of fleece are always dry, and I'm incredibly impressed. Plus, my two seem to love how soft it is.
Also, are you planning on getting a friend for your guinea pig? They need a companion as they're social animals, and I think the minimum size requirement for a cage is around 120.