You can always make a wooden box and line it with foam, cut out for the istrument (don't forget to make it deep enough to have foam UNDER the fiddle as well as around the sides). Simple pine board for the sides and plywood for the top and back should do it. Hinges and a hook-and-eye for closing, or even a belt/strap for keeping it closed, and a simple drawer-pull or screen-door handle will finish it up nicely. Probably cost you $25 for the materials. Use glue and screws for the attaching, not nails.

I'd make the box as a whole, then band-saw it to separate the top from the rest (instead of trying to use thin strips and plywood to make the top separately; you get better alignment and contiguous grain that way).

I did something like this to make my fiddle-and-mandolin-in-one-box case that I never use anymore. Still a nice box, though.

Or just look in local junk shops or on eBay for a fiddle case.

Bob