I would strongly encourage you to make the adapter plates from MDF.
It took me about 30 mins to make all 4 to the correct size, only using a normal jigsaw (don't even have the circle cutting piece)
I have a pair of the plastic adapters for the front door of the 210, you can have them for £10 if you like (RRP £25)
In my CLK i spent ages reinforcing the original plastic ones and they still didn't sound great compared to the "solid" sound of MDF.
http://www.mbclub.co.uk/forums/electronics/69071-clk-w209-stereo-upgrade.html