What the hell are you trying to accomplish here? Your original post says you want to put a 12" L7 in an enclosure that is now holding a 12" RL-p. If that is still the case, I don't know why you want to rebuild anything. The square cutout will be fine and the circular cutout will not affect it at all. What's this talk about a 15" L7? The 15 will not like the 2.2 or whatever cubic feet.
I was trying to figure out if you had some custom fiberglass mold for the bottom and thats why you wanted to rebuild it, but it seems like this is just a basic box.
There is no reason to try and rebuild the box, it is stupid and a waste of time.