Rounded cornerz in the inside of the box and on the port will be very benefitial.
just on the outside, like on the back of the box, would be virtually aesthetic only thats all im saying, id def do it.
Your only gonna run 1500 to each sub right? If you doublelayer each side an innerbrace is unnecesary.
That brace will most likely make you loose decibles and decrease efficiency. Your design is pretty rock solid with no need of a brace.