Love making something for people out of their "own" tree. Best has to be a number of summers back when a white-haired gentleman pulled up in the driveway and addressed me with the last name of the former (15+ yrs) owner of the home forty. I introduced myself, and asked who he might be, only to find he was the child of the owners of this farm who left in 1940. Gave him a bowl out of one of the big poplars that had gone over in a storm, only to find that he and his brothers had planted the trio. He wasn't sure which was "his," but with the other brothers deceased, he felt it had to be the one from which the bowl was turned.
Cottonwood is like the rest of the family of willows and poplars, soft, long-grained, and really fuzzy when wet. Cuts clean and shiny when dry if it's straight, and not too badly if burly or curly. Since it's soft, it'll sand quickly.
Get something high-solids in as a sealer or be prepared to dump oil finish into it until you'll swear someone's pumping from the other side. Which can be pretty if you cut it less than a quarter thick, because the light will shine through.