Litter Box Woes

My poor cats deserve a new litter box. They have been sharing the same box for roughly four or five years. For a short time, I was not cleaning it enough, so they started using the litter catching mat in front of the box as a second litter box when the litter box was full. (I didn't know they were doing this until a week ago). So I threw out the nasty stinky mat. Bought them a new bright, pretty, filtered litter box, a new mat, and a new scooper. The new box has a hinged door to help control the smell and litter spillage. (The old box once had a door, but they broke it off after a few months).
I took Elvis and Maddy down to the basement. Showed them the box. Put them in the box. Showed them how the door swung open and closed. I showed them each twice.
I thought we were all happy. No smelly dirty box. Fresh sleek plastic. Brand new litter. Nice new mat.
Then there was a smell...
An uninvited smell where there shouldn't be a smell.
A poopy smell...
....in the dining room.
At four a.m., Richard woke me up frantic because he noticed the smell (I noticed it the night before but thought it was Logger's gassy behind).
So we searched, me half awake for the origin of the icky odor.
Richard discovered it. Hidden between the wall and the bookcase where we keep the spare high chair cover and the awning to Christian's first car seat (forgot that was there). The cats had made their own litter box in desperation. So we scrubbed and washed and washed and washed and washed the fabrics and the floor (thank God they missed the carpet).
We figured out the cats couldn't figure out how to enter the litter box. The hinged door was a barrier to their sanctuary. Pushing their head against it to go in and out was just too much (don't forget they have used a hinged litter box door in the past). So I removed the protective door.
And I checked tonight: they are using the litter box. Yeah!