It depends what you wanna do with the stationary computer. If you want to keep it secure from outside, then just don't connect it to the internet, neither via cable or wifi.
You could still go online using a system on a usb stick.
The thing is, without internet connection the only access to your data would be through physical manipulation.
With Internet no Operating System can be hundred percent safe. Like the best doorlock can't prevent thieves from going through the windows, through the wall or simply ring your doorbell and knock you out.
So if you want to keep something confidential, don't ever put it on a machine with internet access or that has the possibility to access the internet.
I think this will never change.