Diagral Alarm TAG

I have a Diagral home alarm and I was going to buy an extra keybord tag like this:
http://www.diagral.it/allarme_senza_fili/comandare/transponder_grigio_talpa.html

But then I realized I could just build it for less than 2€.
I bought 27mm round RFID tags 125KHz here
https://www.amazon.it/gp/product/B01BAMK73G/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
Then I just printed this simple case

Oct 2016: added Heart version