I made one from the hardware store for $15. Seals the ball bearing with an o-ring and doesnt leak until spring pressure is hit. True Value sells a ton of springs with different .## pitch wire for different stiffnesses so you can dial it in. Just search for Home depot boost controller you will see many of them.
I built mine for shits and it hits 24lbs on my 3076R right on the nutts and holds perfectly. I have a little too stiff a spring in it right now, becasue the adjustment is about 2 turns from all the way out, so if i put a softer spring in i can dial it down and have more adjustment.
Ghetto as fuck but it works perfectly. I am going to build another one and run a VW pressure silonoid to make a $40 2 stage boost controller.