The PPI PRO104 (Flat Piston Subwoofer) has a 4.25" mounting depth. They're freaky expensive too ($200+).
Thanks for the suggestion, tho. I'm still looking. I've e-mailed Lanzar for the TS parameters for their NEO10 sub. It looks to have about a 3-3.5" mounting depth. Plus, it's only $55.
