It can be built for under 5 bucks. Buy these items. Note the button only has 2 prongs on the bottom- thus creating a simple on/off circuit.
Also buy a Midi cable if needed.
Drill hole in top center to the mount button.
Use a razor knife and carefully cut about a 2 inch splice to open the midi cable cord cover thus exposing the smaller plastic colored covered midi pin cords. Careful when splicing the outer cover to not accidentally cut any of the smaller internal cords.
Pick any one of the smaller cords inside and cut only 1 of the smaller cords in 1/2. Next remove/splice the smaller plastic colored cord cover to expose a small amount of wire on each of the ends of the cord to solder onto any one of the 2 pins on the bottom of the button.
Now you have completed a simple on/off circuit. You will be able to test it before closing the box. If it works you will see the blinking light on the boomerang stop blinking when you push the button on/off.
The last step is to screw the bottom onto the box. You will see when you try to do this the cord is in the way. What I did was I used my drill to simply file a half circle on each length side of the plastic box so the cord would eventually clear and so the cord would be flush. When I filed the half circles enough I then screwed the bottom onto the box so it would properly close.
I then mounted on my board. I dont have a close up pic of the inside of my midid on/off box because the cables are run under my board etc… but I attached a picture of the button and my board.
It is a hacker solution for sure and some may have a better way but for under 5 bucks this solution works well.