My cat thought I had built him a new litter box when I made our garden last summer. To keep him out I made a barrier around the garden using 4 foot bamboo stakes and black plasic mesh (I think it is meant for keeping birds out). I tied the mesh to the stakes with string and made a place where we could easily open the fence to get into the garden. The cat never attempted to get in so this might work for your neighbourhood cats. Good luck with the gardening.