Instead of replacing that silly medical kit lid, what about this?
 

http://www.crutchfield.com/S-sBJYAWBilxz/App/Product/Item/Main.aspx?search=square+driver&i=2066S10L54

You could fab a mdf ring/mount and then cover it with appropriate colored speaker cloth. Is the gas tank in the way down there?

I don't believe you have anywhere near enough room over the tank. Take out the first aid box and measure.

no way!
Find a Plate speaker, and FWIW I don't recommend Crutchfield:(
They push what is popular for the newer cars and won't help older Mercedes car owners. At least that has been my experience!
Several manufacturers make plate type speakers that fit the shallow space under the first aid box on the 123's but watch out when drilling any holes there!
DDH

definitely measure first.. The ring might have to be 3" high for the magnet to clear the tank.

Not enough room. You only have about 2-3" between the board and the fuel tank.

That much magnetic energy might also affect the level sender readings.

I do recommend Crutchfield. I had no problem finding a stereo and front and rear speakers for my 300td. While Crutchfield might not be able to say what will fit directly without any modification, they have plenty of components that will work. Do your own measurements, look up the component data, and figure out what will fit. The archives here are a great help as well.

I only use crutchfield as a reference, they have good reviews and the best online catalog. I buy all my stuff on ebay because I am cheap :)

I paid ~$300 for my current headunit Pioneer HD/mp3 with boston acoustics all the way around.

That wouldn't even cover the head unit at crutchfield.

"mounting depth: 6-1/16""

Seems like its a bit too deep. I could be wrong.

From what little experience I have with Crutchfield, they are not the cheapest, but they back their sales up, and they provide just about everything that you will need, and often stuff that you don't, with the items they sell.

On speaker depths, in my SD, I can just fit a set of Infinity Kappa plate speakers in the raised mounts on the sides of the rear deck.
I'm not sure how much difference there is in the level of the bottom of those chambers and the area below the first aid kit holes, but I agree with the others, I doubt you could fit a sub in there.

I think a sub that close to my fuel tank would stir up all the old funk!

A simpler and easier solution is to mount the sub below and rearward of the medical kit opening. The sub speaker cone need not play directly into the cabin. The medical kit lid could be easily modified into a grille with a dremel and some grille cloth. Solobaric subs take a lot of power to drive them, IIRC. RT

If you have access to wood working tools and have the know how....do what I did. Take out the old shelf and make a new one with whatever speakers you want. I have some detailed pictures about 1/2 way through in the thread I posted on soundproofing.
http://www.peachparts.com/shopforum/diesel-discussion/189355-sound-proofing-126-a.html

The bins on my rear shelf now house a set of Kicker 6X9's.. :D

a sub in a mb diesel? how would you hear the lovely sound of your well-tuned motor humming, and dont forget the whine of the turbo, Sweet Siren of forced air...