
Can you post some specs on what exactly you are trying to build? What exact features are you looking for? Do a google search for handmade shotgun/rifle also instrument/saxophone case. There are a bunch of hits, even plans. The basis of them is pretty simple.
I have to ask though, are you looking for something to use for transport and storage or just a show piece?
BTW, what O/U do you have? Do you hunt or just clay shoot with it?

