They're great for day trips and getting fish on and off the boat easily. Despite my boat having multiple fish boxes and having quite a collection of fancy and non-fancy coolers, the Reliable kill bag I have gets used a LOT.
My Calcultta kill bag keeps ice frozen about as well as my fridge's "crisper" keeps vegetables fresh. That is to say, not at all. Still better than hanging my fish off a rope over the side...but not by much.
I asked Smak plastics about custom boxes once and not to speak for him but basically said the price to build a one off would be prohibitively expensive.
It would be interesting to know what problems your having with your kill bags. As others have said, I can load mine with ice the afternoon before tuna fishing and still have 80% of it left when I get back from fishing the next day and toss it all because I over estimated my ability to catch fish.
For my fish boxes, I had WPS build custom liners for my last boat and on my current boat I made custom liners for a fraction of the cost. (WPS were a lot nicer and better built than mine.) In both cases I can load ice Friday afternoon, fish all weekend and still have 30-40% of the ice left Sunday afternoon. That’s with putting fish in, taking fish out, opening the door each time etc.
For what you are after you have a few options to explore if kill bags or liners aren’t your thing-
Have someone insulate your fish boxes by building a smaller box that goes inside the bigger box and putting insulation between the boxes. This eats up storage space pretty quickly. The other route is to insulate the outside of the fish box if you have access to it.
Build a custom insulated box inside your box. Every time I’ve thought about that it prices out as as much or more than liners and seems like a giant pain to do.
I'm one that hasn't "caught on". To me, kill bags are yet another solution to a problem that doesn't exsist, unless you need to bag a big Bluefin, Marlin or Dorado. I'm sure they work, but I don't need one. Maybe if I caught more fish, I'd feel different. I had one of the orange ones, go Beavers. I bought it in 2019. It kept fish plenty cold, but after a couple uses it leaked cold slime in the boat and stunk no matter how much I cleaned it. I sold it for a great price to someone that doesn't mind a stinky, leaky ice bag. I went back to a nice 100 qt. Igloo ice chest I bought 7 years ago for about $70 at Costco. It cleans way easier than the bag and works as a seat or lunch table. It's not as on trend as the bag but nothing about me in on trend.