Jim, this spring at the Norris show there was a rifle there that had a curved iron lid that was kept in place with two or three oversized iron staples that the lid slid under. It looked darn good. It was on a Southern rifle, and I think it was a modern build, not antique. It would look good in slightly domed sheet brass with rounded front, with a narrower staple as a stop. Just a thought. Depending on the period, brass would be the choice over sheet iron for the lid. Don't know if it was a fix or poor boys sliding box. Wish I took som pic's.
Bill