52 lbs for the flatpack and 30 something for the driver....subs can get heavy
I usually install the driver after I move the box into the room to make it easier on the ol' back. I don't think I took any pics of the duratex now that you mention it. It's black, really black. I'd thin it more the next time for a smoother finish, too. I'll check or take one and shoot it to you....
The museum putty is something I bought for other reasons, works great for a variety of things, including putting on the backs of picture frames that might vibrate from the subs...and its very easy to remove (like fitting it for testing before finishing) not sure caulking would be as easy to work with, but that and several other things I'm sure would work. There's a bit of a gap even with the 15/16" bit I've been using lately, and more so with the 1" before that, to fill. I tried a 7/8" spade bit initially and that was too small.
I buy reels of 250' 12g speaker wire so just attach the speakons to that; I use one at each end but you could go bare wire at the amp end, but I like the secure connection of the speakons at both ends (they lock into place).
One thing to think about, is the 3cft sealed box the one you want? It'll lower the f3 a little with a 4cft box. Or did you consider a ported box? While parts-express only sells a few of the flatpacks the same flatpack manufacturer sells a few others, including some ported boxes, available at diysoundgroup.com. They don't get the bundle with the driver pricing and the boxes may or may not work with the UM-15 driver (I didn't look)....just a thought before you finalize....