Being an avid woodworker, I couldn't bring myself to buy any of the drink holders I've seen for the E34... they either didn't look that great, or they didn't look like they'd work that great.
I built a prototype a month-or-so ago, and have had it in use since then, minor refinements along the way. I have some photos and a text file that describe my design... the final project isn't quite done, but you should be able to get a sense of what the thing's all about. The URL to the photos is below... take a look at the "console" files. There're a few pics of my new tires/rims in there as well.
If there's enough interest I can provide measurements so you can build one yourself. I could possibly build them to order, but I'd rather not get into that business.