Cats make a toilet in my flower box

im looking for some help, I have adopted 2 feral cats that have started visiting the garden since my Tigger died earlier this year, I’ve been feeding them it helps with keeping any mice or rats populations down. Big downside is they have taken a shine to my raised flower bed as a toilet, I spent an hour last Saturday digging out all their faeces and replacing it with fresh topsoil. I also put some chicken wire in on top and just pressed it down onto the bed thinking that they wouldn’t get under it because of its wiry nature. Big mistake they got into the tunnel formed by the wire and used it without digging the usual hole wit her telltale mound of earth beside it. Looks like I have another hour or two today clearing it all again. 

Has anyone any suggestions of what I could put into the bed to put them off, I really don’t want to net the box off completely as I have honeysuckle and jasmine creepers planted in it along with daff bulbs and a few other plants and shrubs.