I travel internationally with 4x5 field gear (wooden folder, lenses, filters, spotmeter, etc.) several times a year.

My recommendations:

Don't worry about having a hard-shell case for your carry-on. In fact, you don't even have to carry your camera backpack on with your gear.

What you do need is a bag that is relatively small and meets the carry-on requirements as to size and weight so you won't have to check it at the gate. I carry my bag on small commuter flights as well, stowed under the seat because it is a bit large for the overhead bins on the smaller planes.

I pack my camera (together with its ground-glass protector) in a thick-walled cardboard box that I have cobbled together myself using a box cutter and duct tape. All my lenses go in similar boxes. The boxes protect on five sides, but are lidless. I've never seen the need for a lid, but you could make one easily as well. These boxes go into the carry-on bag along with other items. My meter has its own soft case and travels in it. FWIW, I use these same boxes in my photo pack when in the field now. Good for protection from small bumps and dings.

I only carry-on the most fragile and valuable items. Important is to decide what you really need to carry on and what you can check. Less fragile items can go in the checked baggage (empty film holders, cleaning gadgets, exposure record book, dark cloth, lens hoods, even filters if you have cases for them and pack them between soft items).

For many years I had a carry-on lined with closed-cell foam (again homemade using an old sleeping pad). I no longer use that, but it is another option for you if you feel you need extra protection.

I often travel with loaded holders inside Europe. In that case I carry about twenty holders plus extra film and an empty film box or two for changing. These go in my checked baggage and my very gracious and understanding wife carries a row of holders in the bottom of her carry-on as well. I've never had a problem just running these through the carry-on baggage scanner.

However, to avoid possible problems, pack cables and other electronic gear in a separate bag in your carry on and take these out and run them through the scanner separately. The only times I've had to have my film holders rescanned was because of other x-ray opaque items in the bag that blocked the view enough to require unloading and running things through separately. I have had my bag visually inspected a time or two. Last trip, the security person was understanding enough to exempt the film holders from a rescan and sent the rest of the carry-on contents through the scanner. The problem was a ball of computer cables, not the holders.

Take a sample sheet of film with you and learn the phrase "unexposed/undeveloped film" in every language you plan to encounter on your trip just in case English doesn't work (Google Translator can help here, but most airport personnel now speak pretty good English). Don't ask for a hand inspection anywhere but in U.S. airports. You just won't get one and you'll slow things down. Make the scanning personnel's job as easy as possible and don't worry about having your film scanned a few times. I've had 400/320-speed film scanned up to eight times with no ill effects.

Hope this helps,

Doremus