I have never found plans for gun cases. I started by taking English cases apart in the process of restoring them. (As an aside, I have restored several English oak-and-leather cases with exposed oak wainscoting that had pine filler hidden below the exposed oak. I just can't believe that oak was so scarce in England to go to that effort to save a little oak.)
The way I make a case is to join the sides and ends by dovetailing the corners, then cut a 1/4 inch rebate in the edge of the top and bottom. Then I assemble everything at one time. I glue the corners and glue in 1/4 inch plywood top and bottoms. After the glue sets I cut the top from the bottom, install hinges and fit the case to the gun. I had a friend cover my cases in leather. Many of the English cases don't have metal hinges; instead the leather cover on the back serves as the hinge. There are lots of tricks for making the wainscot and for fitting the cloth. If you open a case and smell cedar, I probably built or rebuilt it, as I used a lot of red cedar for blocks and spacers.
