"Universal" jigsaw piece, connects with itself on every side

[edit]Done with this. Check the remix, the new interlocking tile. Not universal anymore, though.

This is a first attempt at a blank jigsaw piece that can connect at every side to copies of itself.

The intended use is for scrabble - locking pieces mean they won't get scrambled so easy. Hence the interlocking parts at all sides.

Naturally it shouldn't be used on its own - would be rather boring :-D

I tried to find something like it, but didn't find anything. Either it doesn't exist or