I replaced the serpentine belt & tensioner on my '99 e300d today, thanks to this very helpful thread.
I thought I'd post the pics of the tools I made, just in case it helps someone else later on:
For the 8mm Allen bolt for the cooling fan, I cut a bit off the end of an 8mm Allen key, and welded it to the flattened end of a 2 foot length of steel tube:
To keep the water pump pulley from turning, hacked up some square tube, a couple minutes with the welder:
In use:
Neither one are pretty, but they only took 15 minutes to whip up and made the job a BREEZE.