It took me darn near forever but I finally built my subs. They're not quite 3PI but very close. I came across some cheap flat packs (last two) and they are a bit larger, so the vent changed a bit. My old age, and a shattered foot make handling thick sheet stock difficult.
I've found that the Rolls SX95 stereo subwoofer filter works great with them.
Thanks for sharing your design Wayne.