I just had a (great) idea: imagine a sturdy but still bendable piece of plastic (like the kind used for folders, but a bit thicker), slightly smaller than A2 size, just big enough to fit 2 folded dress shirts side by side. The shirts are held by something like a money clip that's riveted to the plastic sheet. The plastic sheet can be folded over and fastened, effectively making it impossible to crease your shirts even if you just shove the whole thing into a duffel bag.