Prost! uses Arduino Pro Micros as intermediary to allow Midi host to Midi host communication.
Here’s a demo video with Ableton Live, an Organelle, and Norns/Fates. If you’re interested in building your own, check out the Github repo with all you need to get up and running.
Longer demo on YouTube
For now I’m using a simple protoboard, while I further test. In the future I’ll create a custom PCB to make hardware implementation even easier.