Store-bought cages don't give guinea pigs enough room. (Guineas need anywhere from 4 square feet to 7 square feet PER PIG so I know my cage still is on the border of small, and it's twice what you see in this photo.)
First I got some "chloroplast" -- it's that plastic cardboard -- signs are made from it. I called around to sign companies in our area and it was relatively inexpensive. Here are other things I have used (may contain affiliate links.)
I figured out the size I needed, cut it, made sides about 4 inches high, and attached it on the corners.
Then I connected storage grid organizers. Other people have had success using the connectors in the box, but I wired mine together. It is more moveable for me as well as seems to stay together better. I made sure the guineas couldn't eat any of the wire or get hurt on it in any way.
Line the cage with the bedding of your choice. (I put a layer of puppy pads *underneath to absorb moisture, then fleece, and I actually layer this so I can do an easy "cleanout". Wash the fabric by hand to get the worst of it off, then it goes in the washing machine. I have also used Carefresh.)
Of course, you can also decorate your cage, some people even make multiple levels so the guinea pigs can "go upstairs" (just be sure and make it so they can't fall and don't make the ramp too steep!)
This is an inexpensive cage compared to store bought, it will give your guinea pigs enough room, and you can design it however you wish. I've even seen some done in sports decor!