I use one box of baking soda sifted together with one box of cornstarch. (Proportions unimportant; the cornstarch is just to make it less gritty.)
If you like it scented, you can sift in bath powder or put whole spices in the storage jar. One way to get a dispenser is to buy a small container of bath powder, sift it together with the soda and starch, refill the container, and store the rest in a tightly-capped glass jar.
It is a good idea NOT to use soda as a deodorant when wearing silk or wool. I have a silk undershirt that has been eaten away under the arms. One of these years, I may patch it. (Hey! An on-topic remark!)
Joy Beeson