I recently added 3 more litterboxes for my cats because I was having peeing problems. I tried the lower open boxes with the framed sides but the cats would pee over the top of them.

I then went to Petsmart and bought an open box with very high sides but the entrance is very low so even senior cats cat use it without any problems. Storm loves them. Here's the link: Petsmart Litterboxes.

I have also been putting the puppy training pads behind and in front of the litterboxes to catch help catch accidents and litter spills. You shouldn't need to put litter on them first.Thumper will hopefully figure it out and just pee on it. I think that they come with a special scent to them. Good luck.