I am thinking of doing something very similar with the 12” and a HB GPA-100 power amp, to make an active cab I can use with a preamp pedal. My idea is to stretch the side panels sufficiently to accommodate the amp in a compartment on the top of the cabinet, so the enclosure volume would remain the same. Taking the idea a step further, I could have the top, front and rear panels of the compartment lift off to be used as a wedge under the cabinet to tilt it up (one panel would have to be hinged so it could be folded flat to form the wedge). A full width compartment would provide sufficient space to store power and other cables in transit.