One other consideration would be an inflatable GERD pillow. It's hard to tell how lightweight they are, but that might be a good way to pack something down small. If you google 'inflatable portable GERD pillow' a couple options pop up. Unfortunately, it doesn't look like they're really designed with camping/backpacking in mind, so they didn't clearly call out weight and size.

Just a thought, hope that helps!

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