I guess it could be made from DIY PCB kits, I know there are HDMI to SPDIF audio extractors tiny PCB's and there are SPDIF Bluetooth transmitters tiny PCBs, It's just a matter of connecting both SPDIF ends together and have them both powered from the HDMI 5V pin which can handles up to 300mA, The casing can be 3D printed at home. It's worth investigating.