I use a karrimor Sabre 45L rucksack, with a back length of 19 inches. Also a Gransfors small forest axe, also at 19 inches. I find that the best place to secure this axe is inside the main compartment of the rucksack, with the axe head centred at the top, next to the back support. The only modification to the rucksack that is needed is to sew in a small loop of paracord. This is the most effective way to carry any heavy gear, high on the back, and as close to the spine as possible. If you keep your axe head oiled, then all you need do is make a hood for the head, out of neoprene nylon.