Mechanical boost controllers are generally a valve that permits enough air to bypass the wastegate actuator to prevent its opening.
This can be done by calibrating a couple valves to leak enough air to permit peak boost, very simple and effective and inexpensive.
It can also be done with a spring loaded ball valve that closes when the reference boost pressure reaches the point where the valve is set. These too can be acquired inexpensively but are generally simple and industrial looking (not necessarily big - just not bling) then there are the ones made for the car market which are overpriced but do look better under the hood.
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