IKCH-54 Rotary Encoder
This board supports up to 9 rotary encoder switches. The actual PCB has score lines so that you can split it up into two boards of three encoders each if you want a smaller footprint.
Software does the heavy lifting; the hardware merely provides a decoded switch matrix with diode protections.
A note about the schematics
The ability to split the board into two originally prompted me to include a separate CD4028B decoder for each half. However, post layout, I realized that I would never use the second CD4028B, for the simple reason that it's a three-input decoder, and it's decoding into three targets! Therefore, you can go directly to the three targets instead of decoding two bits into three. Unless you really need to save that one I/O pin, just short the CD4028B to bring the three A, B, and C inputs directly to the D1, D2, and D3 (first board) or D4, D5, and D6 (second board) outputs.
Geometry Hints
The board is laid out in a 3 x 3 matrix allowing up to 9 rotary encoders. You can use just the four outside edge encoders if you want to go with the really big knobs.
Revision Z Board
The Revision Z board is just a "shield" that gives a nice cover to hide the electronics.