I lost one of the edge caps to my Square-1 (specifically the edge to the middle layer) and haven't been able to find it. Does anyone have a spare one?
(Sticker color doesn't matter, but if it's orange, it would be appreciated.)

I have quite a few edge caps from one of the original Square-1's. Is this the kind you're looking for or is it one of the new ones?

Show me a picture of the back of one?

Here's a pic of two so you can see the top and bottom.

...by "edge", I meant the edge of the middle layer. Apologies for not clarifying.

Ahaa, that makes more sense. In that case I can't really help you. Sorry.

(sigh) That's OK. Anyone else?

I'd say your best bet is to go ahead and build one youself. You will probably not be able to remove it (without bruteforce), but I dont think you are in luck finding someone who has a spare cap.
You can build a new cap from spare plastic sheeting. If you dont already have that, then you could probably use what you find in CD cases and DVDs.
If done right it will look better than the hole you have right now
Gl!