Closed cell foam here, too. My dog is getting old and needs the insulation and some padding underneath. Besides, he sleeps indoors on a nice cushy bed at home.
I've tried the GG Thinlight, but it is too flexible and wads up too easily. By the time my dog has changed position several times during the night (each time he turns around three times before lying down; I call him my Robert Benchley dog), the pad is wadded up in the corner of the tent. Its only advantage is that it can be rolled or folded tightly to fit into one of the bags of his pack.
I have used a blue foam pad cut to 20" x 30". (Dog is a 75-lb. mostly Lab, but he sleeps curled up in a ball, nose to tail.) More recently, I got a Gossamer Gear torso-length Nightlight pad at the same time I ordered some other stuff from GG. The one I got was the lighter type GG had for a while, weighing almost an ounce less. The current Nightlight, though, is about the same weight as the blue foam. In both cases, I folded it in thirds and carried it in the pad pocket of my pack. Since I use an insulated air pad, either the blue foam or Nightlight work a lot better in my pack to provide lots more support and padding for my back. I could roll it up and tie it to the top of my dog's pack, but it would keep getting caught on brush.
The last trip I was on, though, the dog moved to the foot of my grandaughter's sleeping bag. She has an adult-size REI SubKilo (on sale cheap, and at age 9 she's obviously not going to shrink in the future!). He loved the unused lower part of the bag and she enjoyed her extra-warm feet! Fortunately the shell material of the SubKilo is considerably less fragile than the shell of my WM bag!