Earlier this year (well, last year) I started an extension cab project.
I have a Fender Stage 160 DSP which I really enjoy. Fender released the same thing (well..) as a Head + 4x12 Cab package as well. The amp (circuitry) in the combo is the same as the halfstack. The amp is rated 160w @ 4Ω, 100w @ 8Ω. The stock speaks are a parallel set (2 @ 8Ω parallel - 4Ω total circuit), and I wired the cab up to operate two series circuits which will plug directly into the stock speak jacks (two speaks in series, then two two series run parallel at the amp, giving me 8Ω total). I decided to build the cab using the stock speakers in my amp (G12T-100's) not only to maintain a similar tonal quality in the extension, but also so that if anything ever happened to my combo, I'd have some extra speaks on hand. The way I have it set up, either the stock speaks play, or you disconnect them and you get the extension cabinet instead - It was never meant to be an active six speaker setup.
I based the specs on my original amp (go figure, not such a hot idea!). I used the same size and type of plywood (3/4" Birch, I believe). I dug around the forums here for tips on proper wiring and construction of the baffle and such, big thanks! I purchased some grille cloth from a music store, routed the sides, screwed'n'glued, douched it with a little rattle black and that's as far as it got. Due to some financial constraints, I never got around to putting the finishing touches on it (such as the tolex, rear-mounted jack plates, or handles/chrome corners/etc).
I think the most important part, though, is that it sounds good, and I probably wouldn't have done it without you guys!
On to the good part: Pictures!
Here's a pic of the whole setup with my acoustic Ibanez:
When I based my specs on the stock amp, I forget a couple things. I really had to squeeze those speakers in there. Second (and more importantly, I think) is that I made the extension cab the same depth (front to back) as my amp - bad news for a tall amp rig that's so top-heavy. In order for it to sit on the carpet without me feeling like I was going to knock it over at any given time, I decided to use the rest of my plywood to make a base for it. Eventually I'll paint that too, maybe even route some channels into the bottom of it, slip some cable in there, and mount some fender-style guitar jack/jackplates onto the top of it at the very front. What for? Who knows, why not? (Haha, no, really I was thinking about a discreet fx loop access. Probably won't happen for a while, if at all.)
Here's an internal shot between the baffle and the grille. Notice the attractive glue splotch. A signature touch. There's a speaker missing from the photo - It happens to be laying in a chair just out of view.