Tom shared his 1-3 I2C splitter he made in the project log forum:
One of a series of modular expansion boards for the I2C bus: a 1-3 splitter for branching I2C signals.
This is the first of a series of boards with compatible headers for modular expansion to the I2C bus. The boards all have the same shape and dimensions, 0.925″ square with radiused corners and 1/8″ standoff holes. All can be used with polarized or unpolarized headers that can be soldered on either side of the board. This one is a simple 1-3 splitter with 0805 pad for an optional capacitor. The picture above shows a chain of splitters joining hacked voltmeter displays to the I2C bus of an Arduino Uno.
The design files are available from Github.
Via the forum.