Since the C16 uses a different connector for the Datassette, it needs a dedicated version of the Tapuino. This also impacts SD2IECs as the IEC Serial connector does not provide power, which implies that they must usually be connected to the Datassette port for that, thus we need a dedicated version again. Add to that the fact that I hate soldering Mini-DIN connectors, and the lots of free space available inside the breadbin case to the left of the mainboard and...:

I came up with this connector which has all the signals required to implement internal versions of both the Tapuino and SD2IEC. Both could be implemented on a single board or on separate boards that would pass-through all the signals to a second connector on their left edge. I think there would be enough space to add two boards if they are made slim enough.

There's no need to remember which signal goes where, the back has a handy legend for all the signals:

Something we'll have to think about is how to handle the buttons and leds that these boards would require, not to mention the Tapuino screen. I don't want to encourage people to drill holes in their cases. I guess we'll see.