Flexibility is good. The goal is not for it to hurt. The goal is for the device to give, and then for the resistance to increase as you press in on it. This is different from a bag in that the bag offers initial resistance, and then caves in under pressure. Don't make it too flexible.
Stiffness helps hip rotation.
It should only move about a 2 inch margin under full pressure, no more and no less. The makiwara teaches distancing, timing, and dynamics of breathing, movement and thought when used properly. It is possible to purchase a metal base for an indoor post, mount that base indoors or on a large, heavy-duty plywood plank, and then have your own, personal, portable makiwara. Whatever you use, it should have increasing resistance as you press into it, about a 2 inch give, and it should never, never reach a dead stop suddenly during that resistance. The spring devices you mount on your wall usually fail the third test.