Focus window replacement

OK, so i returned home this afternoon to find that the focus window on my 300/2.8 (non-is) fell off somewhere during the day without me even realising.
Whilst the odds of obtaining an official replacement part is slim (i think the newer 300/2.8's have a different shape), i was wondering if anyone had any DIY replacement ideas?
Perhaps i will look to see if there's anyone out there who could make a custom one.
At the moment i am just using some gaffer tape to cover it, but i don't think this is a very future proof idea.

I think the part you're after is a "ya2-2514-000" but don't see much in the way of hits when googled so as you say, may be out of luck getting an OEM one.

Guess your other option is do some careful measuring of the window area, and have a good look at the sizes of other windows on other similar lenses, and see if you can get something as close to the size as possible (and file/sand it to size if needed)

something like this? http://www.ebay.com …ories&hash=item​4a9f42126e
I have no idea wether the 500mm f/4 window is anything like the 500mm f/4.5 (which seems to be the same part number as 300mm f/2.8 window)

Thanks for the reply.
Yep that is the part number i am after.
There seems to be a russian website that may have it which i'll look in to.
That ebay link is interesting, i will look in to it further!