You already have your chair back, in the form of your pack. It fits your back perfectly! All you need is a comfortable and waterproof sit pad. I use a square of closed cell foam (you can get a large sheet at Walmart for about $8-$12 and cut several pads. The Walmart pad is quite thick.)Then you need something to lean your pack against - a rock, tree or I have even used my trekking poles. Granted, you have to get down and up off the ground.

I know that is harder for some significantly older backpackers. But living on the ground is a basic part of backpacking, including getting in and out of your tent. So as we age, we all should add agility training to our exercise routine. Unfortunately too many backpackers quit due to the difficulty of living off the ground. So sad. In cultures where all people live on the ground, even the very elderly squat, get up and down.