I recently finished this bow up for somebody. Its from some dense red osage that I cut a few years ago. The grain lines really show up in it. The back is mostly concave and has some slight humps and bumps down the entire length. There is a small knot hole in it. This bow seemed to fight me all the way through the build. It's 66" long and 50@28". It is finished with 6 coats of tung oil. The tip overlays are bighorn sheep.
Good luck with your sucker bow. Are you using the sinew for protection on the back or for performance? If you are just using it for protecting the back, rawhide might be easier than sinew. I just sinew backed a 24" bow last week. I tried a new technique on it and really liked how it turned out. It left the back very smooth. You have to use hide glue to make it work. I can send you a link to a How-to if you want.
Knox will work fine. I've used it before. I bet not to many people know that jello is made from animal skin. Black rat skins look great on osage bows.
There is a selfbow shoot in Marshall Michigan every summer. I meet up with a bunch of friends there. This year somebody came up with the idea that everybody should bring a 24" bow to shoot for distance and shoot the course. I've got a few different bows in progress and a few more I want to start. I'd like to get 12-13" of draw out of my sinew backed bow and at least 30 lbs. They will be hard to shoot accurately. I want to make some sort of pistol grip to attach to it so I can shoot it like a crossbow. I'm hoping that's more accurate than trying to shoot it like a normal bow.
Here is the link to the smooth sinew How-to. I skipped the last step with the inner tubes.