That limits your choices, doesn't it? Of course, if you didn't need to be movable, you could make a structure from concrete - Something like an ampitheater. But if you must be movable, I have another good suggestion.

I would make two large panels, perhaps having wheels on the bottom. The two panels would make sides of the horn and the ground makes the bottom - Something like what I use the walls of a corner for in the cornerhorn. If you want a curved flare, then you can make protrusions that narrowed the area closest to the motor chamber, in order to constrain the throat.

Something like this:

For the panels, I would probably build a framed structure like the walls in your home, using sandwiched expansion foam surrounded by reasonably thick wood panels. For installation, bring the two panels together at their apex and have them firmly clamped together. They should be tied together along their length with braces put along the top and bottm, or perhaps with guy wires or rods holding them taut.

I'd avoid building an entire horn structure, because it will be hard to keep it from being "flimsy" at this scale. But the panels probably wouldn't be too hard to work with, and your motor chamber would simply sit in the corner. It can be a box that is shaped like monitor speakers having a diagonal face, and you can load several drivers in the motor chamber cabinet. Even MF/HF subsystems can be placed within the horn if it had a conical flare or a curved throat shape that wasn't constrained too much.