While planning to build a CBG I am reading a lot of theory and I came across a MIT paper about sound hole shapes (https://news.mit.edu/2015/violin-acoustic-power-0210). The paper is for violins, but I think the same principles stand for a guitar. Apparently the more perimeter (edges) a hole is having, the louder sound. Thus the the area of the sound hole does not matter much as the air is hardly moving in the center of it. Long narrow holes takes less space than a round but adds more air speed (louder).
Besides the obvious heavier strings, what other simple things could be done while building a CBG if you plan to play without a pickup and you want it to be loud enough for that?