For soldiers, the number one seat available was their hat. The Victorian world did not believe sitting on bare earth to be healthy. After that, soldiers usually found used ammunition or hardtack crates to sit on. More common civilian folding stools or chairs or regular dining chairs would pop up only if the army was camped for any length of time.
Some chairs such as the five board bench or the one board bench can be easily made. Just make sure if you make a bench, that you use cut nails!