Instead of going without HW, I'd rather build a SeedSigner from off-the-shelf components (can be ordered or bought in store) and remove the antenna.
I have no idea how to remove the antenna, but I guess following this tutorial won't make it so hard. This is what air-gap feels like. Confirm me that it isn't hard. 
IMO it's not that hard. But if you don't have tool to remove the antenna by hardware, you could use follow the guide to disable it on User space and Kernel level instead.
Source: https://github.com/DesobedienteTecnologico/rpi_disable_wifi_and_bt_by_hardware#new-scheme-with-examples-to-disable-wifi-and-bluetooth-from-different-layers