From TalkBass Wiki
This is a foam core/plywood composite build based on the standard 15/6/1 plans. All interior dimensions are the same, but the sides, back and top are made of composite panels instead of plywood. As a result, the interior bracing has been modified to reflect the greater inherent stiffness of composite panels versus plywood. (These modifications are still being tweaked.)
For some background reading on the topic of composite panel building, one can check out the following threads.
Zaclite 210 fibreglass/foam composite build
Zaclite 12/6 Fibreglass composite build
Dug dog's first crack at a foam composite cab
The panels themselves are made of 1/8 inch luan (available locally as door skins) bonded to a 1 inch foam core using Weldbond adhesive. The foam is available at Home Depot - rigid pink sheets. (I've seen other colors.)
The biggest obstacle when building with panels like this is finding a way to join the panels that is strong and durable as it doesn't take much force to chip away 1/8 inch wood which would reduce the integrity of the joint. Also, the amount of wood to wood gluing area is pretty small.
For those reasons, I chose to make a wooden frame around the outside edges of the box. I started with 1X2 pine (I wanted poplar, but couldn't find any pieces that weren't warped beyond belief.) which I ended up machining down to about 1/2 inch by 1 1/4. I then bonded these to the edges of the panels. This provided me with stronger, more durable joints, but added some weight.
Mid range chamber meets side
Rear corner from above
This photo shows how I attached the handles, although it's not the greatest shot. You can't just drive a screw into a foam core panel, so I used bolts, washers and nuts. I had to bend the 2 middle fender washers as there was insufficient clearance.
A look at the woofer/ mid range crossover
Functional cab ready for final finishing. The weight at this point is 42- 43 pounds which is pretty light given that it's the full 3 way version with the passive cross-over installed. I haven't yet finished my light weight grill (botched my first attempt), but I know that it will be very light. I did make one questionable decision in attempting to keep the weight down, and that was using plastic handles instead of metal. Metal handles can be retro-fitted if necessary.
So, was it worth it? Hard to say. I have a reasonably lightweight, kick-ass cab, but if I had to do it all over again, I'd do it differently. I'd make the panels the same way, but wouldn't use the 1X2 wood for the joints. I'd mitre the panels so they create a 90 degree joint and then I'd bond an aluminum extrusion to the outside of the joint with epoxy. I'd use something like this:
The stuff I've pictured is pretty expensive, but I've found plain aluminum angle stock available for much less money. This method should be just as strong, but lighter.
For a smaller box, I don't think there's enough weight saving potential to make the effort worthwhile and I would look to using 3/8 Baltic Birch instead. This should be pretty tough and offers a pretty substantial weight savings over 1/2 inch BB.
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