If you want to work with the ground sensors the best idea is to buy sheets of white/black paper and use them as the ground of your arena.
You can buy large sheets of papers at Schleiper (Ch. de Charleroi 149/151 - 1060 Bruxelles, ).
I had good result with these papers:
- White: CAN5084, 200g 75x110cm
- Black: CAN361017, 160g 75x110cm
Note that the ground sensors are quite reliable, but the e-pucks tend to get stuck if there are "hills" or steps on the ground. Try to use thick paper (at least 200g, the thicker the better) and do not put the sheets too close together or the will bend and the e-pucks will get stuck.