Just keep a buffer budget, so if you don't like what you have you can make the necessary adaptations. These projects will always cost more than you can forsee. I rent also, and use a $10 vent grid from home depot to evay some type of normalicy to the unsuspecting eye. Save the cut out and plaster it back and paint it when you move. Paranoia makes us think everybody will automatically know what where up to, but its the last thought on their minds.