Big dummy heads right there. It's a nice speaker but it doesn't sound the same as an RF-7ii. The center needs to match the mains.
What is your display, TV or screen? Can you get a third RF-7ii in there?
Since you already have two of the THX subs, I'd be tempted to double down and get two more, put one in each corner or wall midpoint. Those are good subs. You're probably going to take a good sized hit if you sell those and start over.
Surely you realize that Pelican cases have a lifetime guarantee against breakage, and that this is void if you alter it. Send it to them before you jury rig it and they'll fix or replace it for free.
Paducah Home Theater's post in Need help with my setup asap! :) was marked as the answer