If you cut a 1/4" plywood spacer, you will be fine.
You can kinda see my rear speaker in this pic...
I used a jig saw with a metal blade to open up the hole. I also had to cut a little out of the rear handle bracket thingie.
BTW, those are Infinity 693.3i's. They have a MASSIVE basket on the back of them.
HTH,
BP