Hi, welcome to PT. I'm so glad to hear that you and Blackjack have each other during this difficult time. I'm very sorry to hear about your sister's passing and hope that each day gets a bit easier for you.

It sounds like you've already consulted a vet and all that can be done is something 'creative' on your end. I don't know if this kind of thing is available online or in pet stores but your best bet is probably to make one yourself. You could buy some velcro strips and some fabric (maybe a water resistant type) and sew part of a wide popsicle stick inside the back of the homemade splint. If you're really good at sewing (which I'm not! lol) you could sew in a little draw string at the bottom and top to cinch it closed and keep it on straight.

One other idea if that one doesn't work is you could take a sturdy harness that is wide and comfortable (like this one GOOBY'S HARNESS) and add some fabric to 'attach' the homemade splint to the harness so it couldn't ever fall off. You could sew three strips to connect the splint to the harness in the front, underneath and on the side. As long as the fabric is comfortable and flexible, it shouldn't be a problem.
(p.s. the pic of the harness is for a dog but my brother bought a small one in a petstore in Arizona and sent it to me here in Canada for my large cat and it works very well.)

One last idea:
You can buy a product at the vet clinic and maybe online called 'Vetwrap' and it comes in all kinds of neat colours and it clings to itself. You could use some soft gauze first, then insert part of a wide popsicle stick and some more soft gauze and then wrap it all in Vetwrap. The only thing you'd have to watch out for is if it's on for too long or if it's too tight you could end up with 'fat paw' and the lack of circulation can be quite painful for cats and creates a big fat paw. But if it's loose enough and changed daily it should be okay I would think.

Hope this helps. Maybe some other PT'rs will have some better ideas. LOL

p.s. I'm guessing that Blackjack is very handsome. I have had a Maine Coon and they are marvelous cats.