Would it be possible for you to stitch together the top seams of the two cushions he likes to lie on, so that he could not fall between them? And then drape the blanket over the two, combined into one, cushion on the back of the couch?

One of our Cockers loves to lie on the top of the couch back too...luckily for us our cushions don't sepereate from the back of the couch so no concern there with her falling in. Hopefully you can figure something out...I'm not really sure...

medium size cat bed could help. I see them at pet stores, Target, etc, Walgreens often has them on sale, you should be able to find one well under $20. Could work. They come in square/rectangle, not just round.

What is a dorgi? Is that a misspelling of corgi, or a blended breed like labradoodle?

Dorgi is rescue dachshund+corgi.

For my 16 pound cat, I use an extra large bath towel; I think they are sometimes called bath sheets. Found one in a color very close to the sofa.

It drapes over the back of the sofa and the back cushions and as the cat lies there, it forms a sort of hammock effect that the cat really likes. I adjust it back to normal every morning. It's large enough that it doesn't fall between the sofa back and the back cushions, and I think the terry cloth helps it not slide as much, as well.

If you have really floppy cushions on the back of the sofa, you could also look into replacing the stuffing with a firmer type of foam (or whatever they are stuffed with).

That's odd, my 13 pound papillon "nests" on the top of the couches and chairs, and never falls in between. Your dog must be more "longitudinal". Does he like to burrow in the blankets also? Maybe you could pick a corner and stack up some cushions on their sides. Sometimes I think they do stuff just to keep us busy fussing over them.

I used to have a "snoozing platform" for my cats - it was a cushioned shelf that screwed onto the window sill. I would think it would work just as well for a small dog.

If you google "cat window shelf" you'll see many versions.