I HAVE SEEN THE FACE OF MY ENEMY: His name is PANEL 4.
When you're dry-fitting all the panels, it all seems so easy. Especially when you've cut your panels on a TS and everything is lining up perfectly. Yeah...add in a ton of slippery-slimey PL, and adjoining panels that when attached to the sides might not be totally at 90-degrees anymore, and suddenly things arent' so easy to hold in perfect alignment. I got panels 3 and 4 (I hate you, panel 4) installed on all 4 cabs. The driver chamber is done. I am confident that all cabs are/will be air-tight. I did not have any large gaps, and what few small gaps (1/16" or less) I PL'd the crap out of on both sides. All interior panels are screwed in from the bottom, 1.25" brads shot into the panels to join them to each other.
I took a lot of pics, so I won't caption each one. But the following comments apply to all these pics:
All panels had screw holes pre-drilled with a 1/8" bit. I laid down a super-generous bead of PL before installing the panel. After screwing the panel to the side, I made a finger-fillet out of the PL on both sides of the joint. Then I ran an additional bead of PL on top of all joints, filleting with the finger. (I gave it the finger....bwuahahahah!). As stated in the plans, the panel 4/3/2 joints are not easy to fasten at all. Practice does make it easier, but expect a lot of brads to bust through the sides , as the pics will show. When I didn't get the angle correct, and the brad would shoot through the side of the panel, I'd just keep shooting in that general area until I got one that seated properly. On the interior panels it doesn't matter if you've got fasteners sticking out through the panels, as long as they don't interfere with the seating of any adjecent panels. There are a lot of brads either busted thru the adjacent panel, or sticking out a little bit. I used probably 30 brads per cab,just on the panel 2/3/4 joint as well as a ton of PL...it's ugly, but it's SEALED.
Oh, BTW, for planning purposes: I have used 2 full tubes of PL at this point, and I haven't wasted much at all. A great deal of PL was used making additional fillets to ensure everything is airtight. Absolutely necessary? I don't know...I'm not that an experienced cab builder. Worth the expense? Absolutely. A couple of bucks worth of PL stretched over 4 cabs to ensure air-tightness? PRICELESS.
PICS. Ask questions if you'd like but I"m sure you can figure this out. The propane cylinder was used as a counterweight to keep the assembly from tipping over onto my head. Ask me how I figured out "I should counterweight this panel!"
Tools of the trade.
) and he asked what I was building. I showed him the Sketchup model, explained what they were and he couldn't wrap his head around the horn concept. He kept asking "How does the sound get out if the speaker is all closed in?" I explained. Twice. Deer in the headlights look. So I moved on to the efficiency of the design. More deer, more headlights. So I really dumbed it down and said "It gets really loud with a really small amplifier...like the size of a paperback book, the amp could be!" He suddenly understood it all. LOL! Ah, the non-techie population...as long as they hire me to knock the fences down in their backyards, I'm OK with them. 
) And, being that the panels for the T60 dwarf these, I think you've got that covered. See, most architects start with a 1,200SF house. You went and built the Taj Mahal right off the bat. Everything else is cake. 
This is AWESOME. Check it out! Gotta love the ZZ Top background music too. 
