Wayne doesn't/hasn't come around, haven't seen much activity here in years. I just happen to stumble by.
The design you are asking about is nearly 8-9 years old, if you haven't ordered the parts yet.

I would highly recommend you check out the TriTrix design at partsexpress showcase.
There is a smaller MT version you can find here under TriTrixMT:

http://www.geocities.com/cc00541/index.html

Curt is well known and he will answer your emails and he is a real nice guy too.His design is more current,it uses the round frame 5 1/4" dayton. You won't regret it.

but to answer your questions:
I would assume there is some stuffing like poly-fil how much I can't say since he doesn't specify.

The crossover is almost always mounted in the box.

I don't think that design has any braces the cut list doesn't show any and it's a pretty small enclosure.

The grill will cause "some" interaction but I doubt it will have any significant impact.