Yep, thin plastic sheets will work fine as dividers. It's a common solution to isolating spreading plants.I currently have these two planted in a relatively new tank and this is also my first planted tank.
I have the hairgrass in the background with carpet in the foreground. I notice that the hairgrass is rather invasive and wants to creep over into the foreground area.
Is it common practice to place vertical flexible pieces of plastic in the gravel that separates the roots?