I've had to make my peace with just having to hit a store on the other side of the trip. Fortunately, baking soda is cheap enough that you don't have to mind throwing it away.
You could also conceivably make your own pills, which at least in my experience nobody will question: https://www.amazon.com/Cap-M-Quik-Tamper-Capsule-Filling-Mac... (I don't make my own pills, but I've carried around a variety of pills that look like they were made that way.) But for baking soda that's probably a little elaborate, plus I don't know what happens if you take baking soda that way rather than in water.
You could also conceivably make your own pills, which at least in my experience nobody will question: https://www.amazon.com/Cap-M-Quik-Tamper-Capsule-Filling-Mac... (I don't make my own pills, but I've carried around a variety of pills that look like they were made that way.) But for baking soda that's probably a little elaborate, plus I don't know what happens if you take baking soda that way rather than in water.