ok it took awhile but i finally have some dimensions for you. keep in mind that these are approximate dimensions...having carpet on it may have thrown off my numbers, but not enough that it should matter. The overall length is 50 inches in the drawing, but you could fit 51 maybe 52.
also, the blue pieces are a different thickness than the rest. The box was made with 7/16 plywood, and the blue pieces are either 5/8 or 3/4 partical board. Sorry im not sure, but with carpet all over it I couldnt measure it precisely...and I couldnt remember. Only reason why i used the thicker wood there was because i ran out and went over to my neighbors house..my best friend..who just happened to be building his own sub box on the same day with partical board.
Also keep in mind, this box wont work unless you cut out the back carpet and plastic, and speakers. And just so you know, my speakers are not enclosed nor is the amp...but the subs are sealed. Oh and the amp isnt screwed down...it has tis little lip on it which i shoved into place under the top piece of remaining gray stock carpet...aka its not going anywhere. And the entire back wall has extreme dynomat to reduce rattle...although I had none to begin with.
Hopefully i didnt forget anything. If you have questions or think my drawings are confusing, just ask me here and I'll try to help.
EDIT: for the speaker holes, i just made a template by looking at the back of my infiniti's