JP, it looks like you're getting off to a good start with this early advice. Now that you have your necessary enclosure volume, you can play with the dimensions until you get a shape that works for you (don't forget if you go ported, you'll also have to subtract the volume of the port, ideally). --Nevermind, just re-read that you're using a
PR and have it accounted for already. Don't forget to use lots of bracing, especially on the larger panels, and subtract that volume as well. Do you have to make this a specific size to fit in somewhere, or did you have a shape in mind? I'd second the opinion of putting the driver and
PR on opposite sides of the enclosure, but I think I've seen a few downfiring subs with the
PR on one side. Keep us updated!