I just took the midgate out, traced it on a sheet of MDF and went from there, I didn't really record any dimensions. It probably wasn't the easiest way to go about it , but it worked, I used the first sheet as the gate, Had to put a ring around the outside of it to get the lower hinges to screw back in, then had to add 2 blocks at the latches on top to mount the locking mechanisms.
As for the sub box, I just bought one for 2 12s on ebay for like $40. I would have had more in material and carpet than that to build it myself. I stripped the carpet off the front , re sealed all the joints, then just screwed it to my flat midgate that I built, cut the holes to the size needed for my subs (I left the holes undersized when mounting the box) addded some vinyl and carpet, then terminal cups and was ready to rock.