I have been wanting a hardware wallet for Bitcoins and Alts for a while, but I cannot justify the cost, especially living in Australia, as the postage to the middle of "No-where" certainly adds up. I found a great little project, based on the Trezor offline wallet over at https://www.pitrezor.com/2018/02/pitrezor-homemade-trezor-bitcoin-wallet.html The instructions are very easy to follow.
I remixed mobi_electronik design of the keychain, as I really like that one and have been using it for a while.
This remix uses the cheap Ebay 0.96" OLED 128x64 blue screen (AU$3.70), two buttons, small MicroSD (2G) and a Pi Zero. My buttons are huge, but all i had laying around! 12mm sqare, 5mm hole, 6mm depth. This case should fit most small "tac" style buttons easily. I have extended the covers over the HDMI, camera and power as they are not needed for this project. Follow the hardware setup from Yannick's blog, it's very easy to follow.
Edit: Made a slight change to cover the MicroSD slot, to make it a bit more secure. Also cut the text right through the case, would probably look good with some back illumination, or just coloured card stuck inside.
Feel free to modify for your own needs. https://www.tinkercad.com/things/fyXPoQDAAQ7-pi-trezor096-oled/editv2
Oct 2018 Edit: Created a different lid to match the original style of Trezor. The buttons are side by side. Both versions are in the download, so you can choose whichever.
Print flat side down on both pieces