Well my intended baffle is on a 70degree angle, and I want to have a rectangle port. Can I make it a slot port even though it will mean one end of the port is angled? If so, where would I measure the length from? the middle?

Kickerboy ran a slot port box with angled baffle as you described and the back of the port was simply on the same angle aswell so that the port is the same length at any level position. I don't think it would be ideal but what else can ya do, simply run a rectangle port but then there is a chance that it will hit the bottom of the box, or you can just run a flat faced box.

Never thought about making both ends angled. Thanks . The angled baffle is so the box isnt as high, meaning more air can flow between the boot and cabin.