We have finally completed testing and production of the ELAD FDM DOU to Hardrock-50 interface kit. The unit can be plugged directly into either the radio or the amp and uses a cable (supplied with the kit) to connect to the amp or radio. The kit also includes a 'Y' cable so you can still connect other ELAD peripherals. The kit including the interface board and all of the cables is available here:
https://hobbypcb.com/products/hf-radio/fdm-duo-interface
The Hardrock-50's Transceiver must be set to FT817/8 in the menu and no changes are needed to the ELAD's set up as long as you have the latest version of ELAD's firmware. I know that some older versions do not send the I2C band data but I'm not sure exactly where the break is. If your radio supports ELADs preselector, it will work with the Hardrock-50 interface.