Robert, depending on the size of the diorama it can be placed under the seat or in the overhead. The height cannot exceed ten inches for under the seat and the width and depth could be 16 inches and 14 inches respectively. The rule of thumb about carry-on luggage is that all of the dimensions cannot exceed 42 inches (i.e. in this case, 10 "H+ 16"W + 14"D = 40"). You could go wider to eighteen inches and still fall within the safe dimensions.
I have custom boxes built for my dioramas with a sliding panel so it can be shown to airport inspectors if they request it. Typically, they do not after it passes through x-ray. The box construction is your choice and I typically use birch as it is strong and lightwieght. I put a leather strap handle on it.
That's about it from my experience and I have been traveling with dioramas for years like this without ever having a problem.