trapezoid porting?

im designing a truck box of sorts and have the rear perpindicular to the bottom and the front sloping. the mfg for the subs reccomends a slot port (8w7). my question is, how could this be accomplished, and is there any calculators for a tapered port (which would be the easiest, but im not sure if that is an option)?   would fabricating an elaborate port to rectangular dimensions be the best route? would a circular port REALLY not be an option if the mfg doesnt reccomend it?

The type of port doesn't matter, at least in my experiences. Circular ports are a little easier to make. The only draw back, that I have hear, of circular ports is that if its not tapered you can get more port noise. I just built a box of my aw1205Q with 2 circular ports 4X11.25 inches, and the box sounds fantastic. But all that matters is that you should make the port strong, and have the proper volume.
Is the top of your box flat, or does it come to a point?
If it’s flat you could create a rectangular port dropping in from the top.
If the top weren’t flat your best bet would probably be to create a rectangular, or circular, port that comes in from the front, but make sure you compensate for the slat at the front of the port.

the top is flat, yes. but im concerned with the tapering of the ports from the wide bottom to the narrow top, as ive seen no calculators that take this into account.

As far as I know there is no calculation for what you want to do nor do I know what the effects would be. Why dont you just not tapper the port and only make it as big as what the opening at the top is. Or port it out the side closer to the bottom if you need more room for the port opening.

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double-secret reverse-osmosis speaker-cone-induced high-level interference distortion, Its a killer