Just leave it open.
The idea is that under boost the added pressure on the second port would help hold the wastegate closed and then when the proper boost pressure is reached a solenoid would switch it to atomsphere so that the spring can be compressed and the wastegate opened. When the pressure drops below the desired boost level the solenoid switched back to pressure and the waste gate will snap closed.
You’ll get better boost response and faster spool if you have the right hardware to make it work.