We stopped using sandpaper for smoothing wood on field courses years ago. Sandpaper is sand, or grit, glued to a piece of paper in a thin layer. It doesn’t last very long, which precludes it from being taken on long trips. A simple alternative is to take a piece of fabric (denim or cotton duct works great) and a handful of sand, then use that to smooth the piece you’re working on. The fabric holds the sand. The only thing lacking is the glue, and if you need to hold the sand more stationary you can mix some lard or other fat with the sand and it will hold it together (this can be useful for improvised sharpening). It’s easy to transport because you only carry the fabric. You could also wear a pair of gloves and palm the sand.
Another sandpaper alternative is using shards of broken glass to shave wood. Less portable, but it’s hard to go anywhere where you can’t find an old beer bottle.
There are usually numerous creative substitutes for things we think of as necessary. The challenge is in recognizing them, which can be difficult even if the solution is right in front of you.