Something you might already know but worth mentioning. If you decide to use a sheet of aluminum and cut two holes, I would suggest boxing it in so the sheet is 1/2" to an 1" away from the rad but sealed off as best as you can. This will allow air to be pulled through the entire rad. If the sheet is lying flat on the backside of rad with holes cut then you are only going to pull air through those holes.
Whether or not you need the aluminum I guess depends on how much of a cooling problem you've had. Could try without and see how it goes.
I'm running two electrics, set up as pushers in front of the rad. They have their own shrouds with rubber seals and cover the entire surface. The rad is a 26" 4 core champion, never overheats on the hottest of days, even when stuck in traffic.