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This! I have a "lap desk," which serves the same function as a pillow. This is an example from Amazon, but you can find them in lots of places and prices can range widely. Big coffee table books can serve the same purpose.
So far, it's been great. Super lightweight. Pretty stable. And can't really beat the price.
Edit: just looked at the listing again. Looks like they don't have the gray color in stock new. But they do for the other colors.
You can Velcro or bolt it to a lap desk which will be way more stable and yet mobile