Another issue is that all the plans are drawn for current production devices, specifically, those made by Eminence and JBL. As such, all the plans I have are optimal for their intended woofers, which may or may not be a good match for the Beyma 12cx. I would recommend that you enter the woofer's parameters into PiAlign and let it calculate cabinet dimensions for you.

I'd be happy to send the plans for a specific three π or six π model, but I think it would be best to do a little studying first. Download the PiAlign program and read the "readme.txt" file contained in the distribution archive. It is only a couple of pages long, so it won't take long to read and will familiarize you with how to run the program and what parameters are required. You'll use familiar T/S specs, with one exception, Qd = 1/Qts. Other parameters Vas = Vad and Vas = Vad, but all this is described in the "readme.txt" file, so I encourage you to read it and run the program. If you have any trouble, feel free to ask.