you could practice putting sinew on them
keep in mind that just because your draw is 31 inches does not mean the bow is made for that,,or will hold up to that draw,,
each bow is different and should be drawn as such
half the length would be a good rule of thumb to start with,,I suggest longer
a heavy bow requires a good piece of wood
start there,, if a self bow,, make it 70 inches for your draw and see how you do,,
designs are infinite,, but a good self bow will last for years at the proper dimensions,,, that is my suggestion for a starting point,,