Yes, 1.2 cubic feet is more optimal.
Yes, I'd add some bracing and would also line it with No Rez unless you are really trying to do the whole thing on the cheap side (which is fine).
You can do a sand lined box on the cheap and get really good results too, as you can get a 50 pound bag of play sand for about $4.00.
If there is room in your budget then the Apex Sr. is the best of those amps.