Sinew is the best for ERC,but I wouldn't risk a possibility of wasting lots of time sinewing on a questionable piece. I only use sinew on primo pieces of wood. You could try a hickory backing,but those are some serious looking violates knots. Does it look better from the other side? And how thick is it? Your pic only shows the inside of the tree close to the heartwood,and it might be cleaner on the other side closer to where the outside of the tree was. It might just be better for veneers in glass,but I'm not a glass guy.