My initial space requirements aren't that far from the 3 PI turned sideways. Most table/low cabinet furniture is 30-32 inches tall. I can make the box 31 inches, an inch taller. I would like to maintain a 15 inch width for the front baffle. That would leave 18 inches for depth. Not a huge change from your design, provided sideways works. I agree with you. A sacrificial box is the way to go. I'll build one out of MDF that has removable panels, and a couple of different baffles. So the first trial will be a box of 31 X 15 X 18 placing the drivers height wise near your original design. I prefer to place the vent on the back. If it doesn't measure well, I'll add to my scrap wood pile & try again.