I just got a custom-made one from Modular Marine last week (66" long with 2 external boxes), start to finish about 3 months but that included 2 rounds of template shipping to match existing holes drilled by the tank manufacturer.
Very happy with the outcome so far.
Exotic Marine Systems is another that makes custom-sized overflows but I wasn't thrilled with the way they do their internal slots. I think they would have more business if they gave customers a choice. They have explained the advantages in YouTube videos and I get it but I feel it's easier to lose fish/shrimp with their system. Perhaps I'm wrong, but I wanted not to worry with an old-school design.