I'm with Red B. I take a wet rag (get the water as hot as you can stand to hold) and rub gently on the "outside" of the piece to bend, lay it in place and start working it to conform. Don't hurry, and as you keep putting slight pressure on the piece, continue to moisten the outside with the wet rag. It will conform nicely . The top of the Decathlon boot cowl is a pretty gradual curve, but be careful as you get to where the top drops off to become the side, this much sharper bend will take a while to conform. Ammonia isn't really needed as much as patience is.

Balsa does have lignin (the 'cement' that holds the cells together), but much less than other woods.