I don't know jack about this stuff. I was just going to copy the original but use MDF instead of PB. Anything I need to know? Can I screw it together and seal the seams? Does it have to be covered in that felt stuff?

screwing it together is fine. MDF is great wood for a sub box but use a min of 1/2", 3/4 would be preferred. Use a thick run of glue in all the seams before screwing them together or use silicone on the inside to create an airtight seal. And you can cover it in Christmas wrapping paper if you like there is no rule on that.

If theoriginal is a sealed box and has a polyfill stuffing, dont for get to duplicate it. That stuffing is used to acoustically increase the size of the box. Dot put much more than they do. At a certain fill point the acoustic size begins to diminish rapidly.

Use wood glue at the butted seems, screw together with drywall screws (easiest and quickest). You can predrill with a very small drill bit. Seal corners (from inside the box) with silicone. Fill with polyfill only if the original box had.

Does the speaker have to be positioned at an upward angle? Can it not face straight up or horizontal?

a new sub box, I live in Birmingham and do this stuff for a living, plus it would give me a chance to talk to another AUDI guy. You could come down on a sat morning and we would be done in about an hour and a half, might take ya longer to get here i guess but road trips are always fun, plus I want to see that front end of yours. let me know

Marc

Assuming this is a real sub crossed over below 200Hz, these thing are nondirectional and the mounting of the driver doesnt matter, except for issues of rigidity. Generally the shape of the box isnt critical either, rigid, non-resonant, and well sealed are the most important properties.

As the your next question... The size of the box should be set according to the properties of the driver, so assuming the original maker knew what they were doing (not always a safe assumption, unfortunately) yours should be the same. The box volume should load the driver at its lower resonance. Mismatches will result in tubbyness (I almost forgot - some like it this way) or overdamped sound. When building a sealed box from scratch, I either make it a little large with a movable panel to set the volume based on driver-in-place tests, or make it a bit small and add polyfill to acoustically enlarge it as needed to matche the drivers resonance. My favorite sub box isnt a box at all but a length of Sonotube - EXTREMELY rigid. My last project wasa vented compound loaded design and when I measured the enclosure the box loss was below my tables. By the way I dont usually build subs for cars - big PITA, I have built a few for home audio/theater.