I donāt know what would be a good wood, but I think pine would not be it. Pine isnāt all that durable and I suspect would break down fairly quickly. The surface would dent when struck with knuckles, and it splits easily. Pine is typically what is used in breaking demos.
Some types of oak can be somewhat brittle and will crack, but I donāt know if that would be an issue under these circumstances.
In North America, hickory is a tough and durable wood. It is often used for axe and maul handles, and there is a safety issue in that when it breaks, it does so gradually and the fibers tend to hold together so it doesnāt simply fly apart. This is an issue for things like staff work that involves impact/partner training. Again, I donāt know if it would be an issue as a makiwara.
I think the makiwara plank on which the striking pad is mounted is typically tapered in thickness. This creates flexibility that may not exist in a plank of uniform thickness. Perhaps start with a hickory 2x4 or larger, figure out how much of it needs to be secured into the ground in concrete to make it firm, and have it extend about five feet up. Of that upper five feet, thin the plank from the middle to the top, so it is half thickness at the top. Fasten the striking pad/rope wrappings onto the top 8-12 inches.
Iāve never made one, but this seems reasonable off the top of my head.