I faintly remember seeing these “piggyback converter” boards, was it on Reddit? Do you still have the link? Tagging @Adair21 in case he can/will comment.
We talked about this on another topic – I’ve thought long and hard about it, but I don’t feel like i have the necessary skill for that yet 
Or do you mean the Convoy 3x21D with Adair21’s piggyback?