Sitting down is the thing nobody warns you about. A donut cushion is a straightforward fix: put the opening where the pressure would otherwise land.
Two sides: One side is the donut, for pressure relief when you sit. Flip it over and there is a place for the gel cold pack, for when cooling is what you are after instead.
Inflating it: It auto-inflates in about 30 seconds. No pump, no batteries. On the very first use it may want a little extra air. Blow into the valve to fill it all the way.
The cold pack: The gel pack is reusable. Chill it, then set it up on the cold side of the cushion. Follow the instructions that come with the cold pack for how to chill it and how long to use it. Those numbers belong to the pack, not to guesswork.
The cover: Machine-washable. Take it off the cushion before washing. Let it dry fully before it goes back on.
In the box: 1 cushion, 1 reusable gel cold pack and 1 machine-washable cover.
Materials and dimensions are not published for this cushion. If you need a specific size for a specific chair, check the pack. Nothing about early recovery is convenient, but being able to sit down without bracing for it is worth 30 seconds of setup.