yeah i was just going to say try it out as is before modifying.
if you feel like the cone is moving too much - then you can add wood to the inside of the box to take up airspace.
I actually would just use a skill saw and cut the bottom piece of the box off completely - and then trim down the box -
and re fit the bottom piece you cut off. ive done this numerous times to boxes ive built previously and did not like
or just built new. Im a total wood jew - and I recycle my own boxes in to diffrent boxes ... lol cause im that cheap !