Perform the bleeder T modification first, then connect a 12', 1/4" diameter hose to the open end of the bleeder T (use the 90 degree T mod for the most boost). Run this hose to the cockpit (if desired). Connect a brass valve (available at hardware stores in the plumbing section) to this hose. Adjust the valve until the desired level of boost is obtained (close the valve for stock boost levels, open all the way for max boost). Make sure that the valve has a large enough orifice to not restrict airflow. You can test for boost loss by opening the valve all the way and note the boost level, then disconnect the valve from the hose (in the cockpit) and see how much boost you make. They should be identical.

Note that when partially open, you can hear the air hissing out. If this is bothersome, run another hose off the end of this valve and route somewhere away from you.

Note that the longer the hose is, the less boost you will hold because the pressure will start to open the wastegate.

Thanks to Jason Knippel for coming up with this idea!