My sister’s collie Glory is 12 years old.
Technically Glory belongs to my sister, but we refer to her as a time-share dog.
Her original owner was our mother. Sadly she had a stroke when Glory was only a few months old.
My sister and I took Glory to obedience school together. 
We trained her to be a Therapy Dog. We took her to Mom’s nursing home everyday. Sadly Mom never recovered.
From the beginning it was understood that Glory would become my sister’s dog.
I tell you all that so you will understand why I do so much for and with Glory.
Years ago I made Glory a pretty, plump, plush, purple, princess pillow. Its no longer plump. 
I’ve been reading the book “Caring for your aging dog”. One of the things they recommend is a egg crate foam mattress.
I posted a request on Freecycle for some egg crate foam. This is what I received.
I used the green foam (it is the thickest), cut it to size, replaced the original padding in the pretty, plush, plump, purple, princess pillow.
Glory gave it her seal of approval.
Our time-share arrangement works well for both all three of us. I have a dog anytime I want one. My sister has a reliable dog sitter anytime she needs one (she travels quite a bit). Glory gets all the love and attention she could want.
When I finished the dog bed I offered the remaining foam on Freecycle and someone asked for the rest of it.
Hello Frugal Spinster I found you through Creative Savv. I have so enjoyed reading your blog and spent some time in the last week perusing your archives:)
You and Lili must be my twins. When I saw this posting I thought of ny 11 year old lab who is just slowing down a bit. You are brilliant with the ideas you learn about and apply to your situation. Years ago I had a mattress? That looked like a a giant piece of bubble wrap.I had purchased if for a camping pad.Got to see if I gave it away, or if dog beds are in my future also.
Thank-you for posting this great frugal idea:)