The instructions that came with my carbon filter said it was better to pull the air than push it. It's hard for me to visualize the filter you described. Just remember that the air will flow through the path of least resistance--that, is it will flow around rather than through your carbon filters (and even make little air tunnels through your individual bits of carbon if it's not packed right) whenever possible. So if your pvc square thingy has any air space or leaks it will greatly reduce effectiveness. You're putting the brita filters inside the pvc square, right? If so, you could try "great stuff" expanding insulating foam (like $6) and cut off the chunks that expand around the holes in your brita filters.
Definitely buy that ona in case the smell gets too potent. I'm 100% for having spare chemical odor control. I even tell all my friends who smoke to keep some ozium on hand just in case of an emergency. $9 could save your life someday!