I had my oil level window come out ( see trip report "Las Vegas to Bloomsburg 2011), and have been reading of several other BMW owners having this problem. Mine came out after 20 years - so, not a bad record. The service manager of the BMW dealership in Birmingham, Andy Carroll - came up with this fix after learning that there were no windows in the US. I did have this fix come off, didn't check for tightness after a very cool morning - did manage to get it back on and finish our trip. Here are a couple of pics of this item, I will now keep this in my parts stash on future trips.
Still waiting to get a replacement window from BMW, however another K owner had one and has sent to to me, I'll send him one back when I get a new one from BMW.
I believe he got this part at Home Depot, might help someone else with this problem if a window is not available and on the road.