i'm sorry to hear about your retriever's failing health.
i did have some thoughts come to mind about that. i don't know how practical this would be & this is just a rough sketch (not all the construction details are worked out), but i just thought i'd share this thought w/ you.
the spreader bars in the picture are like i use on the head end of my hammock. they don't stretch the hammock into a flat, unstable bed, like the hammocks you buy w/ spreader bars. they just spread the top edges, but still allow the person (or dog) to lay deeper in the hammock, beneath the center of gravity.
to see pictures of my hammock w/ spreader a bar, click on my gallery. i think they will be around page 7.