Maybe not an accessory EVF, but something like a Hoodman loupe would seem to me to be the least bulky way of getting an eye-level viewfinder.
I have a Ricoh GX100 and find that the accessory EVF makes the camera a bit bulky and more ikely to snag on it's way out of my jacket pocket.
The Ricoh EVF is a blocky thing that takes up the hotshoe, it would appear from photos that I've seen online that most of the similar units made by other manufacturers share the same general description.
|Fangorn Forest by cand1d|
|Yosemite Falls with Moonbow by Jonathan Shapiro|
from Best Landscape of the Week 4