So I tried two methods described in these knee comfort threads over the weekend.
1. Strap between legs that rested against the tree. While this was comfortable because it allowed my knees to not have to be pressed into the tree while sitting, it did create pressure on the outside of my legs pulling in. I used a double length climbing sling that was doubled up to make it a single length. I figured using this I could adjust the length depending on the tree width. It was tolerable, but I'm not sure how comfortable it would have been for an all day sit.
2. Two steps for my knees to rest against. This is definitely the ticket! It was way more comfortable than anything else I've tried, and surprisingly it did not provide as much pressure as the tree did. I expected it to be more pressure focused on a smaller surface area, but I barely felt the steps at all. I did use an OCB, but since I'm not standing on these I', going to look for an alternative option to attach them to the tree since it does need to be as tight. I was worried about them being in the way when maneuvering around the tree, but it did not seem to be an issue.
Also, all of this was done while wearing knee pads. I like to have them for when walking around the tree and putting a knee into the tree for stability, or for when I'm on the ground and need to kneel down. The steps method would not work without knee pads unless you used something on the steps to create cushioning.