Moin Walt,
i have build 2*15" BRs. With 10 kg total driver weight, the total box weight is 30 kg. Measures 80*60*65 cm. My trick was to use 13mm plywood and 24*48 mm massive Wood, we call "Dachlatte" to stiff out. The "Dachlatte" has nearly no weight...
This works very good, so my conclusion is stiffing out is much more important than 18mm birchply, cheaper and lighter too. The benefit of 6th order BP is, you need no heavy speaker grille and less handles to move the lightweight box, sometimes the BR holes are enough.... Making the Box even more lightweight..
(That`s part of my fun division :wink: i learned a lot about weight an cost. It`s very easy to build good sounding cheap speakers if truckspace and pro-finish is no problem)
But i would never substitute even this orange monster with BP 6th order boxes. I know about the second Displacement minimum and its benefit, but they sound always ugly. :cry: Do you have to use a Spaceship ? Than it would be ok :lol:
If you can read German, look here:
http://www.paforum.de/phpBB/viewtopic.php?t=29459&highlight=leichter+autodach
And have a look at Sound-Kliniks SK-Bastelhorn. In Simulation 100+ dB/1W from 50 to 200 Hz, in single use ! Ok it`s a pipe resonator @ 50 Hz (in single use) but there is no resonator above. And no port-turbulences.:
http://www.paforum.de/phpBB/viewtopic.php?p=293745&highlight=#293745
Gruß