This is really a simple design, and I'm surprised that I haven't found something like this earlier. It took me about 20 minutes to design this from scratch. The principle is simple and should be able to be seen easily from observing the design.
I've printed this, and you should remove and flex it immediately after completion.
I printed it with an Ultimaker 2 (now off the market) at 100% infill. Works down to about 50% scale and should work at any size larger than that.
NOTE: Use the flat one. It works better. Also, the smaller you print it, the harder it is to flex it apart.