The main difference between this and all the other firmware I could find is that configuring the key mapping is really easy, and its debouncing actually works.
It does require something with built-in USB host support like a Leonardo or a LilyPad. And since I'm using actual C++ flow control it does need a little more EEPROM space than most firmwares, but I mean, as long as it fits, right? You're not gonna be running an OS on this thing.