Cage What to line cage with?

Most people use either Coroplast to make a plastic tray or a heavy duty shower curtain and run that up the sides. Some people do a flat floor insert by wrapping a board or cardboard in the shower curtain so it's waterproof. You can either insert it inside the grid walls or just laying a flat sheet of linoleum under the grids.

If your cage is on the floor you can just put the grids on top of a larger piece of whatever (shower curtain, linoleum, coroplast). It also means you won't have a grid bottom. Those options wouldn't go up the sides of the walls so stuff will fall out of the cage. The easiest answer is make a tray out of coroplast. Typically people make the trays anywhere from 3"-6" high walls. The higher the walls the more stuff stays in. The lower the walls the more viewing area you have to watch your piggies.

Yes, you need to put something absorbent between the fleece and the liner. People typically use uhaul pads (the recycled denim ones, not the padded ones) available at the uhaul store or online. they are 7.95 for a giant pad which you can cut down. Some people also use disposable stuff like puppy pads under the fleece. Some people use old towels. Uhual seems to hold the smell, absorb the pee, wash and dry the best out of all the options I have tried.

In all honesty, if you have a stock size cage it's much much easier to buy the coroplast pre cut and pre scored from the cage store. They look a lot nicer than any of the ones I have attempted at home and come in a ton of colors.

https://www.guineapigcagesstore.com/coroplast-bases

So what is chloroplast exactly? I'm sure I can find some somewhere near me, but is it sold at hardware stores or what?

home depot sells it in white, I think it's around 13 for a 4x8 sheet. Sign makers also carry these. They probably have more colors and you can get it precut from the sign maker. They use them to make all those campaign signs that you see sticking in lawns all the time. Similar to corrugated cardboard but it's all plastic.

Coroplast is your best option. I buy mine from sigh shops, as my local Home Depot's do not carry the large 4x8 sheets and are WAY too expensive buying tiny sheets.

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Ha. 4 of the 5 reviews for the Home Depot coroplast are from guinea pig people.

This link will tell you how to cut & prep the coroplast:
https://www.guineapigcages.com/howtometric.htm