Don't modify the dimensions of the box or the port - If you want another loudspeaker, I'll send plans for another speaker. The Thermionic Series three π and the Theater Series four π both work well on amps with low damping factors, such as low-power SET amps.

As for mounting the HF horn on top, I suggest that you move the midwoofer up so that the distance between it and the tweeter are the same as called for in the original design. It's also important to maintain the same front-to-back spacing as well.