It's probably time I got one of the panels off and took a look for myself. Z4C_ers suggestion of a homemade weathershield pocket is something I'll explore further. I guess the key issue here is making sure it doesn't create a source of resonance.
BTW the Z3 never had water sloshing around in the doors, but there were signs of minor corrosion that looked to have resulted from moisture. Strange really as the window seals looked fine and the car was garaged overnight anyway.
@PBondar did you consider the Audison K4M as an alternative to the Eton? It also claims to be a plug & play solution and I suspect from looking at photos it might have a more tapered backside(!) On the other hand its a bit more expensive and I can't find anything online that reviews or compares them against each other