Ley's 12 Pack Cat Deterrent Mat, 6 x 8 in Scat Mat with Spikes, Keep Cats Away from Plants, Indoor Furniture & Outdoor Garden Fence – Pet Repellent Barrier

This is exactly what you need and what worked for us. If you put the soil a little lower in the pot, you don't really notice them, and you can cut to size. We originally tried foil and toothpicks, but they're unsightly and movable. Bonus points for washable/reusability.