x1 NFC Tag (they are cheap on amazon or aliexpress, 30 cent/each, if you ar not sure about the type for the compatibility, search "NFC tag android")
x2 Bitcoin NFC faces normal size 3D printed - Diameter ~42.2mm
You can also print a coin and while it's printing (if your printer supports pausing it's better) stick the nfc tag into it, this way the tagwill be embedded inside the coin / object.
To write the NFC tags you need a phone with NFC, we made an Android App that makes the process easier: http://bitnfc.org
Windows Phone version coming soon!
otherwise you can use
"NFC Tools" for Android (play store) or a similar app
to manually save the private key into the coin
Dogecoin version is being developed at: dogenfc.com (soon!)