Well, I've done the math, several times, if each of those tubes is 2 inches on the inside, or 3.5 on the outside from the MDF (Making them ALMOST scaled in the pics) it'd be between 35 and 34 hz. If it raises to 38 or 39, it's still lower than any crossovers I have.
From what I can tell, exact port length isn't important. If you're off 1/2 inch with ports that long, it'll only change the frequency so little that I doubt I'd notice the difference.
I'm actually thinking of doing this with PVC pipe and joints, just kinda caulking them onto the box and painting them. Yay for chrome. *grin* I really need a digital camera.