You can build a ball and spring type with about ~10 in parts from the local hardware store. Instructions are here. If you want a 2 stage version, you use 2 of them with a solenoid to switch between them (or 3 or however many stages of boost you want).
try the turbo xs unit for a mbc, all the DSM guys and WRX guys are use them and I myself have installed a few on cars out here and as far as adjustability if you want you can run the lines (vacume) all the way to the inside and still can adjust boost.They sell for $95.00 starting...
TurboXS also makes an MBC with 2 settings that you electronically switch between in the cockpit. I have trouble with solutions that run a hose to the cockpit as one advantage of using an MBC is you can keep the hoses as short as possible for the best boost response.
I installed a Grainger-type MBC and am not impressed. My boost is everywhere, except where I want it to be. The MBC is mounted directly on top of the actuator, so the total vacuum tube length is less than 8 inches, however I get 3-5 psi spikes, no matter what I try to set the pressure to be. And on the tall gears, the boost creeps up! For me, it has not been a "set it and forget it" like I was expecting. It does have a vent hole, so I still don't understand why it's spikes and creeps. Any suggestions?