I've been looking for a hard case that would be adaptable for all sizes of hammered dulcimers for quite some time. I think the following EXPO cases will work. They provide a durable shell around your dulcimer accommodating your instrument in its soft case so when you arrive at your destination you don't have to tote around the extra weight of a hard case. The EXPO case is a two part shell held at the desired thickness by the straps shown above. Additional foam would have to be added to keep the instrument snug and protected particularly on the bridge face. I wouldn't worry about the back of your instrument especially if it is in a soft case. The EXPO case expands from 6" minimum thickness to 10" thick so you have room to add whatever extra protection you'd like. When sizing a case to your dulcimer, make it fit snug lengthwise as there is plenty of protection around the edges as the case is double thick there. Although not a certified flight case (ATA), I think this case will provide adequate protection to allow checking the instrument with a greatly reduced weight.
The EXPO 40 and 46 will be alllowed as checked baggage although both these cases still exceed the 62" (length + width + height) maximum size that most airllines accept without extra charges so you may have to pay an extra charge to check. The EXPO 52 is 83" total which makes it oversized as well. At least one airline (Delta) has a cutoff at 80" which means that they would not accept this case as checked luggage at all. Most other airlines will accept this size but you will have to pay the usual oversize charge.
I recommend you check with your airline about size restrictions and costs to check before purchasing this case.
Read additional information on flying with your dulcimer
The following are the EXPO case characteristics.
High-density polyethylene plastic for strength and impact resistance.
Overlap lid design keeps contents secure at variable depths. The overlap lid design allows the lid to telescope over the base, creating variable internal heights. The lid rests on top of the contents and the whole bundle is secured with the nylon straps and buckles.
Telescoping models range from 6 inches to 10 inches deep
Withstand temperatures from -80 degrees F to +180 degrees.
Solvent resistant plastic will not blister or peel.
Heavy-duty nylon-web belts with quick release fasteners.
Recessed 3" easy-roll wheels for easy handling
These wheels are not large enough to roll over deep carpet but work fine for any hard surface.
Heavy-duty spring loaded handles.
Colors vary from silver grey to black. I'm afraid I don't have any control of what color you might get.
Designed and constructed for long term durability and reuse.
Overlap lid keeps case contents secure while keeping the container weight to a minimum.
3 available sizes with variable depths will accomodate almost any hammered dulcimer
There are 3 models available:
EXPO 40 40"X26 1/2" 16 lbs
EXPO 46 46"X22 1/2" 19 lbs
EXPO 52 51"X26" 21lbs
The above measurements are inside dimensions. Add 1" to length and width to get the outside dimensions
All models are $285 plus $50 shipping (comes from CA)
Delivery time would be no more than 2 weeks. If you are interested, pick a model and send me a check for the full amount.