No problem! Rockford, Alpine, and JL all make great sounding shallow subs under 3.5" deep if I remember right, it just depends how much your willing to spend ha.
On a side note, don't use plywood to build the box. It'll kill the sound quality because it resonates so easily and it won't hold up long. Either go with 5/8" MDF if it is under like a 300 watts per box if you really wanna save space you'll just need good bracing in the corners, or 3/4" if it's over that. Or you can use 5/8" birch too, it's lighter and from what I've heard stronger than 3/4" MDF.