A flat piece of rail isn't going to work because the contour of the barrel changes, going from wider to narrower. The rail would not sit snug, nor would it be level.
I had the same problem with my Rossi 92. Rossi sells weaver scope mounts for their 92s but they have been out of stock for many months, with no information as to when they will be in stock. A lot of people are waiting around to put scopes on their rifles. A guy on the Rossi forums made one and sent it to me.
I don't know if you can see, but a scout scope is pretty far up front, the taper of the barrel makes mounting a scope anywhere but the receiver more complicated. OP you might check on the Henry forum to see if anyone has a solution for you. I too prefer scout scopes, they are so much faster to use.