The way I got all of mine to use a "box," which is actually a small splash pool, to potty in, was this:
First I found what they liked to pee on best. It turns out to be straw. They cannot resist straw, especially when fresh.
Then I put the straw in a place outside and started teaching them to go there.
Then I put the splash pool in that spot with the straw and with a little persuading and focusing on times when they really had to go, got them to use it. Treats being offered after every successful production, natch.
Then we moved and I set the pool up in the enclosed porch. They were already familiar with it by that time, and it wasn't long before all of them were happily doing their bidness in the potty pool on our late night or bad weather toileting runs.
I've heard of people buying turf for their dogs on porches as well, to put in a container. So if she prefers that, it might be done. I'd try the straw, I've heard from others that dogs go nuts for it. Either way the big thing is to break it into manageable/teachable chunks for her.
And although she obviously needs to be able to get up and down stairs, and Doug's point about BG control is a good one, it might be a very nice thing to have the large blind diabetic dog not need to go down stairs and up them every time she's gotta go. Or even outside. I can tell you that when it's not a good time for me to go outside with mine, for whatever reason, I'm very very happy that I made that effort. My issues are different.... small dogs, dogs with no hair, dogs that don't like weather, and living in coyote/skunk country... but I think in the end there are always going to be times when going outside is not desirable.