I think that is what you are thinking of, so here's how I do mine. It works very well and it is very easy and simple to clean!
First, I have my little litter box (it's like a little bin my mom found at the dollar store-cat litter box is a good one to use too!) I have a single guinea pig since she doesn't get along with other pigs, so when you have two guinea pig the bigger the litter box the less likely they are to fight over it. I dump all of the bedding into a trash bag and then I spray the litter box with 50% vinegar/50% water solution, and then wipe it with a paper towel. Here is my clean litter box!
For the very bottom layer I use puppy pads. My guinea pig doesn't chew very much, so she has never chewed the puppy pads (I also am very good at covering the puppy pads with the bedding!
) Newspaper works too, but since you are using puppy pads for under your fleece you might as well use it for your litter boxes!
If you guinea pig starts to chewing the puppy pads, even a little bit, then remove them and change to newspaper since the plastic on the puppy pad can be harmful for them. Here is my litter box with the puppy pad. I fold it to size and then fold the plastic edges underneath so there is only a little plastic showing.
Then you add your bedding! Simple!
In this next picture I have Carefresh in the litter box which is a great bedding, but I usually use Back-2-Nature bedding. I ran out of it a couple days ago so I used some leftover Carefresh I had!
Sorry this is such a long post! Anyways, I put my litter box under my pigs hay rack but you can also just plop a huge mound of hay in your guinea pig litter box so they can tunnel under it. Here's my guinea pig reaching for a piece of hay in her litter box!
My guinea pig discovered that it is super fun to jump in her hayrack and poop and pee all over her hay, so that is why there is that little yellowish thing on the side of it. The other side is pushed up against the coroplast.
Depending on the height of your litter box it shouldn't be a problem for your guinea pig to jump in a out of it. I wouldn't add a grid either because that will take up some running space, and one of my guinea pigs favorite things to do is jump in her litter box as she does zoomies around her cage! I think jumping in and out of her litter box is her favorite part! Silly pig!
Hope I could help you! Good luck with your first time piggies and thanks for adopting!