I know some people use those colorful foam puzzle pieces. I wouldn’t trust those with a bunny who loves to chew though, but there are some that are just like them but are a hard plastic. We use them in the garage with the weight equipment. When I used to have Mimzy caged in an x-pen, I used a big piece of hardboard. & in order to keep the x-pen from sliding around, I just drilled holes in the hardboard & zip tied the x-pen to it. However, the downside is that the hardboard can begin to get warped if your bun pees on it often.
If you’re looking for something softer though, there are things called carpet tiles. They basically come in sqaures & you piece them together as you like. The cool thing about these is, if your rabbit destroys one of the carpet tiles, you only have to replace that one tile. If it pees on it, you can remove just that 1 carpet tile to wash & then put it back. They often come with little sticky tabs that go on the under side to stick to the floor so they don’t move around.