in bbp6, model a large vented box w/huge, long port. make sure to enable vent resonance peaks in the graphs. then place an internal baffle that slants into the corner within the enclosure volume. you will then have 2 chambers. model the chamber with the driver in it as the volume of a small vented box. the area of where the slant meets the corner should be the port area and measure down the center out towards the port exit for the port length. make sure to match the tuning of the two enclosures. so if the large vented box is tuned to 30 htz., tune the small chamber and slant distance to corner to 30 htz. as well. when you put a slanted baffle in a box and do not extend the port, the box reacts like a double chambered bass reflex enclosure (DBR) with the internal port being the area from the slant to the corner and the port depth being the thickness of the enclosure material. the resonance of the exit port will resonate at the internal port, in this case being the slanted area. so what happens is that the box will see the compliance of the small vented box but will be tricked as if its a large DBR vented box because its tuned the same. analyze the graphs. you'll notice that the box will have a lower, flatter impedence and limited excursion while having the frequency response of the larger enclosure.
