Has anyone tried this yet? If so how did you do it, please post pictures if you have them.

I'm curious as to what size of off the shelf sub with box would fit or if I need to have an enclosure custom made.

It would be great if there was enough room there for one 12" sub and subwoofer amp.

Any suggestions are welcome.....

JBL rejected that as an option for their stealthbox. There is not a great deal of room beneath the floor.