hey, i want to build a 1x15 +horn bass cab

i read the cab tutorial, and had a few quick questions
in terms of cab dimensions, i wanted to make it deep, does anyone have any suggestions on how to do this without increasing the dimensions of everything, or should all the dimensions just be odd

also, if the speaker and horn are both 8 ohm each, does that make the cab 4 ohm? do i need a 16ohm speaker and 16 ohm horn to make the cab 8ohm? i only know ohms in terms of cabs and heads (head handles 4, my other cab is an 8, i want this to be an 8)

also, in terms of porting, i see a lot of cabs that have the "slit" on one edge, as in the speaker is essentially isolated, and there is a small row that i assume goes to behind the speaker, like this one here

is this design better or worse than a port hole constructed out of pvc probably

also, any recomendations on a quality bass 15", i have looked at a few, but they are all advertised as PA speakers (i know there isnt really a difference, but still wondering)