I'm in the process of making a cupholder in the ashtray area of my 300SD. I'm writing up a how-to (also on my blog). Right now it is curing so I am breaking this into two parts. Here is what I have so far:

Taking the old ashtray apart revealed a couple things...1 - that it was incredibly simply designed with a plastic tray and pop-up lighter on a spring. 2. The front wood was just glued on. 3. It was asking for a mod.
You can see that it simply slid on rails, here is the outer housing:

Therefore, you could make a thin board-like structure to slide in and out--this was the idea behind my design. I wanted to keep it pretty simple, no metalwork, no hard cutting, nothing too crazy. Here's what I've done so far:

1. Cut out a template from cardboard
Traced out the circles, I used a coffee mug as a template...then cut those bad boys out! Here is the rough idea - it should look similar to this once finished (without the tear):

Next step was to cover everything in either fabric or fiberglass. I had an old flannel sheet lying around and flannel happens to be PERFECT for this type of application. A lot of custom audio guys use flannel soaked in resin as a base for custom speaker boxes. It isn't perfectly flat, but will be decent once the resin soaks the fabric:
Next, mix up the resin/hardener to specification (1/10 can of resin = 1/10 tube of hardener) and thoroughly soak the whole thing, however not leaving any excess resin floatting around. Stick a sheet of release paper (I use wax paper, it isn't too bad) under it so it doesn't bond to whatever surface it is drying on. Here it is, soaked in resin:

Right now it is curing. Part 2 will be following a good night's sleep and 8 hours of work.

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth