...if I could find ~2 inch speakers and custom make a frame to hold them in the center cavity at a slight outward angle, I bet it would sound better as the stereo left & right channel sound bounces off the windshield.
Hi Bob,
and I bet that you would loose.
In order to perceive a good stereo sound your head should be at the tip of a triangle and the speakers at the base, that is, your left ear closer to the left speaker and right ear closer to the right speaker. Even if the waves bounce of the windshield you would not be sitting in that sweet spot but if they are reflected by the door windows you might. The problem with that arrangement, however, is that the waves are no longer in the proper phase to each other and worse could cancel each other. Also your ears can only distinguish left and right sound for high frequencies (above about 300 - 1000 hz) that is why you can put a bass (woofer) anywhere in a room and you need only one. But those need a large volume, much bigger than the available space in the center of the dash.
There are plenty of 2" (and smaller) speakers available but these are all tweeters so you would have to place a subwoofer (bass) speaker somewhere unless you prefer a thin "tin can" sound.
Your 280SL was designed for stereo speakers in the kick panel - no need to cut any metal - so why not go that route as Waqas suggested?
But if you really want to use the center space then use it to put "stingers" there. Go to:
http://www.mcsquared.com/mono-stereo.htm#demoand mouse around the top graphic on the right and you'll see what I mean.
A fellow in Germany put his stereo design on the German web site.
Go to:
http://www.pagodentreff.de/artikel/einbaustereo.htmland download the PDF file. His solution was to put the a new amplifier/radio under the seat. It is controlled via infrared (IR) signals and can accept any MP3 (iPot) and other sources. The subwoofer he put into the space below the rear parcel shelf (still covered by the carpet) and the 2 mid-range speakers behind the kick panels. But the real kicker is the placement of the tweeters!
He put one 21mm (0.8") tweeter in each of the fresh air supply ducts behind the rotating chrome grill opening - isn't that cool!
Copy it and you have enough work to do for the next 3 months before you come to the PUB and we can listen to the results
Good luck!