OK, my $.02
Rectangle vs round, little if any difference.
Flaring/routering the openings can have a slight difference (usually a very subtle difference in depth.

But I was convinced a slot port is actually more of a hole cut in the enclosure with no porting at all (similar to a bass guitar cabinet). This is what I am using for my (2) 24" subs. I ran several examples, scenerios, and hole sizes using Bassbox pro. So far the preliminary tests (running VERY long speaker wires from my home sub amp) sound pretty darn good.