Mike, WA6ISP has a small PCB that will add 16 more I/O Lines. It runs on the I2C lines, along with 5V and earth. It is 1.750″ x 1.100″ and has 3 address lines, so you can run a several of them and they won’t interfere with the SI5351 (which also uses i2C lines). The chip uses the Adafruit Library for Microchip I2C Expander. Mike has a few built up versions and a few bare PCBs. Email him if interested using his BITX20 list email address.