Coroplast is a corrugated plastic (and brand name) that has become a popular go-to for open road fat track builds. It's like cardboard, but plastic. It is also the recommended material to use with the Magtrack Open Track Project by 3DBotMaker.
You can buy Coroplast from various places. Certainly online but also in retail builder stores like Home Depot and Lowe's. Overall, it's a pretty cheap material and easy to work with, no unlike cardboard.
Recently, the guys over at Hot Car Track shared a video with their review of Coroplast and how they're using it on their track build. Give it a watch if you're interested in some thoughts on how to use Coroplast and how it works with cars.
I have never used Coroplast for any track build but it's on my radar for when I get around to building myself an open track. I like the idea of cheap materials that are easy to manipulate, as I'm not a big planner so when I mess up, I don't feel like I just burned a pile of money.
My concerns with Coroplast is connectivity with retail tracks (orange, crashers, etc) and then the turns and curves. Coroplast isn't incredibly flexible so it's great for striaghts but might not be great for the curves (hence the worry about connectivity).
I also don't know if there is any generic Coroplast out there...I'm sure there is.
If you have used Coroplast for your track build, share your experience and maybe some photos of your build.