Add the return in the room on a side opposite of your positive pressure vents.
If you just want a quick/easy fix, the opening through the walls should be helpful so long as the existing vent is not along the same wall.
If this system is one of many, and its positive pressure vents are all in rooms with doors I highly suggest closing the central return, and adding a return to each room, and putting a damper in each supply duct for fine tuning the flow. I have done this in my last two houses, and the comfort difference in the rooms is outstanding. I imagine this is more efficient as well, since you are likely keeping these rooms cooled/heated while sleeping and not loosing temp controlled air to the central area that is not in use. Make sure your thermostat is in one of these rooms, or move it there if not; preferably to the master.
Easiest solution: move your thermostat to the master bedroom as well and you will always have the temp you want in your room. You may freeze or roast others, and it will cost you more in utility consumption, but it will be easy and get the desired result.