I found Uncovered via a link in the Blu-ray.com forums thread for custom covers. I mainly print alternate cover art when the original packaging won't fit in my shelving (Criterion's Godzilla set, for example), or when the packaging is of a sort or quality that doesn't protect the discs adequately (though in either case, I of course do still store the original packaging separately). I'm currently looking for individual case covers for the Spider-Man 1 + 2 + 3 UltraHD release that has the "book" style case with paperboard sleeves.
I'm sure now that I know of this place I'll be looking here in the future as well when this sort of thing comes up. I did try designing my own cover art a couple of times, but I never got a result worth using. If I ever try again and create something worth using, I'll be sure to share it here too.