Lots of times Goodwill, Salvation Army, Catholic Charities, Ebay etc.,. will have old empty wooden boxes from everything from cigars to shipping boxes to jewelry cases, etc. that with some TLC will shape up into fine muzzleloader display cases. You don't have to be a woodworker to refinish them and add a lining and they are usually pretty cheap. I have done this several times.
There are also tons of wood boxes with lids hinges etc On Amazon as well for under $20 sold for knick knacks, crafting, etc, some even finished and others that wlth a minimum of finishing touches would work well.