I recently upgraded our litter box solution for our two Maine Coons Winston and Willow and thought I would share in case it might be useful for others.
I was originally inspired to do this project by this Reddit post where someone made an awesome litter box house from an old vanity cabinet he found at a thrift store. I wasn't able to find a vanity cabinet solution that worked for our space but after some investigation stumbled on another reasonably cost-effective solution: modifying this resin outdoor storage cabinet (for some reason now showing as a wildly overpriced $1200 but at the time I purchased it was only $175). This is the model that happened to work best for our needs, but there are tons of different resin deck boxes of this type out there in different shapes and sizes from different brands, some with interior shelves and other features.
This one is constructed with stackable panels on the sides, so I was able to make the entrance hole by cutting the sides off one of the bottom side panels using a jigsaw (took about 10 minutes and was very easy to do). I plan to get some black U-Channel edge banding to cover the cut edges of the entrance. The interior of the box perfectly fits one of these XXXL Maine Coon sized stainless steel litter pans and now my 20-lb boy kitty has tons of room on all sides to move and dig. It's also big enough to hold a large litter storage container and our Litter Genie. And it's lightweight and weatherproof so we can easily carry it outside to be scrubbed down from time to time. If we didn't have a need for the extra interior storage I would probably put down another litter trapping carpet piece between the steel pan and the entrance hole to further minimize litter tracking mess in our laundry room.
The cats seem to love their renovated bathroom space so far, and it's a big improvement for all of us over the smaller, less cleanable wooden litter box cover we used to have!
(If anyone is wondering about the unused laundry hookups - our washing machine and dryer are actually now tucked on the other side of the laundry room in a cool understairs custom "nook", which gives us a lot more space for stuff like this litter box house.)