I should point something out that's pretty important with this build. It's certainly not a design flaw, but this may make it a little easier.
The cutout for the terminal cup should be moved up a little bit for the mains. After you build the basic part of the enclosure you'll need to drill the holes for the crossover inside the box. It's done prior to installing the inner chamber.
The drawing pretty much has the crossover butted up on the bottom wall. The problem is that it will be hard to drill the screw holes for the crossover in that position, unless you have a drill bit extension. Without an extension, you can't drill a hole 1/4" out from a corner, your drill won't fit. It would also be harder to install the screws as well.
So if you move the crossover away from the bottom piece of mdf about 1", you can easily get everything lined up. Here's the problem with that, by moving the crossover up, it will be too close to the termianl cup cutout that you'll make on the back.
Here's a photo showing what I mean:
That's the center channel, and that's about where the crossover sits inside the box (1" away from the sides is 2" away on the outside of the box).
It's not too big of a deal for the center channel because the terminal cup is positioned differently.
But as you can see, if you are building one of the mains, you need to turn the terminals length wise, which would not fit properly. So you basically need to move the terminal cup UP on the box about an inch I believe. I'll double check my measurements, but I believe I moved the crossover up 1", so I needed to shift the terminals up that far.
The good thing is that you will still have access to the rear of the tweeter heatsink if you only move it about 1".
It's not a huge deal, but I could see it ruining someone's day if them missed that. Luckily, I looked a few steps ahead as I was building, so I didn't have an issue. And as mentioned, it really isn't a design problem because a drill bit extension could be used to drill the holes and install the screws.