Flying with your instrument
Sarah Garland of Alaska has worked out a nifty way to adapt a archery bow case so that it can provide an excellent travel case for my 3/16/15/8 Travel Chromatic model with 7/8" string spacing. The case is a molded plastic, SKB brand, trapezoid-shaped case with ATA locks on the latches."The case is lots heavier than a soft case, but much lighter than any other ATA case I've seen. I suspect it will still fit in the overhead bin of big jets, but if it gets rejected as a carryon, it will give the dulcimer good protection if it has to go below in cargo.

"I tore out all the padding that came with it and refurbished it to hold my dulcimer and stuff. I used rigid styrofoam, and then covered it with a stretchy velvety fabric -- it looks like a "real" instrument case inside. The most important thing is to fit the styrofoam so that when the case is closed and turned upside down,the dulcimer can't shift, the weight is on the frame of the dulcimer, and the bridges don't touch the case. It isn't gorilla-proof, but I think it's fine for gate-checking. The only downside is that it could really use some wheels. I strap some skateboard wheels on it for going through airports. The case fits my James Jones 3/16/15/8 Travel Chromatic (with 7/8 inch string spacing) perfectly; it would probably fit a typical 12/11 but may not fit a typical 15/14. I love this case. I made cubbyholes to hold all my dulcimer paraphernalia (which I take out and put in checked baggage for flying, but it's handy for everyday use). AND, after years of envying guitar players with hard cases covered in cool stickers, I can now decorate my case with stickers!"

The case is available from a number of different sources.

I ordered it from Cabela's because the shipping cost was MUCH less there.
This link for the same case has more details of the dimensions:

This outfit also has some bow cases which might work and are less expensive.

The first step is to pretty much remove all the existing padding in the case. You might want to leave in some of the padding along the edges.and than modify it with some extra styrofoam so that if it gets turned upside down, the weight will land on the frame rather than the bridges. The most work was cutting and fitting the styrofoam.

This photo shows the interior of the case showing the blue and pink styrofoam and the black spray foam she used to customize the padding of the interior.

This photo is the same view, but after she covered it with stretchy purple crushed-velvet-like fabric.

This photo shows the case with her "stuff" placed in all the little custom niches she made for it. The stuff fits in cutaways in the foam underneath the dulcimer, in the space that's required to fit the tristander brackets in. When the case is loaded and closed, the dulcimer holds all the stuff in place so nothing can rattle around.

This photo is a view of the dulcimer plus stuff in the case. Fits like a glove

"After using this case for nearly a year, I can say that I love it. I'd recommend it to anyone who is getting a Travel Chromatic dulcimer. It's my daily-use case. I only resort to my soft case with backpack straps when I know I'm going to be hauling the instrument a long way and want the lighter weight and ease of the backpack case. "
Sarah's Update 2012:

This case DOES fit in the overhead of big jets. However, I doubt that they'll ever let me carry it on again. The end of free checked baggage caused most people to carry on their suitcases, AND the airline I always fly (Alaska Airlines) has tweaked their schedules so that nearly all flights are full or nearly so. That means they aren'twilling to let let me take up space with my dulcimer case, which exceeds the official dimensions for carry-on luggage. They DO let me gate-check it. (One bonus to gate-checking is that, so far at least, they don't charge for it.) I've never checked the dulcimer in regular luggage, so it hasn't
been subjected to riding on the conveyor belts and possibly being thrown around. It's best to keep it out of sight when checking in at the front desk, because if the agent there sees it he/she may try to tell me I have to check it. It fits through the x-ray machines in security screening. Sometimes they just x-ray it; sometimes they open the case and swab it.
I've had it swabbed twice. The first time, they asked me to lift up the instrument myself. The second time, they nearly arrested me when I moved to lift the instrument. I got off with a firm scolding that I was NOT to touch anything while they inspected. Don't you love TSA?