Why are bass drum reso portheads circle-shaped? That's gotta be the hardest shape to cut with a box-cutter. I'm gonna make mine a trapezoid. Unless you tell me not to. Then I'm REALLY gonna do it. Then I'm going to make it a quadrangle that's almost-but-not-quire a trapezoid.  No but seriously, can you think of a single acoustic reason why?
i've never heard the word 'porthead', ..... google doesn't seem to recognize it too well either.... just came up with the band 'portishead'.... anyway, i'm guessing you mean the hole in the front head? i think the reason that's round is to be aesthetically coherent. the drum is round. and i don't think they are cut by hand. it's certainly done by a machine. also it might fracture easier if it was angular. those circular shaped ones seem pretty solid