Even if your ride doesn't come with factory speakers in the rear deck, it doesn't mean you shouldn't put some aftermarket drivers back there. Building a baffle is relatively simple. All you need are some basic ingredients (e.g., MDF, fleece, resin, body filler and vinyl) and some tools like a saw and router. Here we'll show you how to build a custom panel from scratch for CA&E's classic BMW project car.
The original rear deck panel on this Bimmer had already been removed. That's good news for us as it cuts down on demolition time. As you can see, the metal tray that separates the cabin from the trunk allows plenty of depth for a set of coaxials. | 
Before continuing, we have to unbolt and remove the seats. Here we got rid of the headrests (who needs 'em?), which means getting busy with an airsaw. The metal brackets are sawed off on both sides. | 
With the area clear, we can make a cardboard template for the base. |
Using the cardboard template, we cut out the base from 51/48" MDF. Check for fit back in the vehicle. | 
Sand the edges of the MDF before routing it to get rid of imperfections and ensure a smooth routing job. Also, knocking down the big stuff with 50-grit now cuts down on sanding later, when it will be more difficult to sand the beveled edge. | 
For the sake of good looks, we'll route a 45-degree bevel all around the edges of the piece. |
Next, cut the mounting rings for the speakers from a piece of 31/44" MDF. | 
Router the edges to ensure they're the same and finish nicely. Create plates to match out of 11/42" MDF (not pictured). Use these to protect the rings later when coating the rear deck with resin. | 
Marking guidelines of the centerpoints on the MDF base makes it easier to center the two rings where we want them. |
I like to tilt speakers at a 30-degree angle < less isn't enough and 45 degrees is too much. To get this angle, cut stilts from MDF with the tops sliced at a 30-degree angle. Attach them to the base and rings first with wood glue before nailing them down. | 
Sand the sharp edges of the bevel all around to soften the shape. | 
Notice that there's a gap in between the rear deck and the seat. Fill this in with another piece of MDF. Again, grab some cardboard and create a template of the area before cutting out the MDF. |