I made one a few months ago and used a recycled fleece blanket, and it worked fine. Not to flat, and not to fluffy!

I made a few. The first I made were poly as it was only batting I could get hold of. I made these quillows for young nephews to take camping. They did find the closed cushion and poly difficult to cope with. I made the next batch without closed cushion with tabs joining the sides. The 5 year old found this easier to use.
Then I started making them with warm and natural. I also started using a bag or a loose cushion cover to hold either a normal size quillow quilt or single quilts. The warm and natural was thinner than poly and easier to put in cushion but warmer. I love to keep quillows in the car for children to sit on or lap quilts.

Any batting you want is fine. I also quilt the "pocket", so it can be used to keep toes warm in a lap quilt. DH loves his.

I've made several with low loft poly batting. Some were cross hatched and some were either tied or tacked by machine. In some I included a pillow for the pocket.

Couldn't imagine making one with polyester. I used warm and natural because I didn't want it fluffy.

I've made a number of quillows and my daughters and grandchildren love them - especially for travel. I think I used whatever I had which was probably high or medium loft. I tied all of mine. A great gift and they go together quickly.

That's just what I need. A lap quilt with a pillow into which I can put my feet. My feet are always cold when I'm not active.

That's just what I need: a lap robe with a pocket for my feet. My feet are always cold when I am not active. I have the pattern; now to just get at it.