4" or 4x6 plates fit nicely in the rear brackets. You'll have to drill mounting holes or make some brackets to mount them in though. I don't think you'll every find a speaker that just bolts right in. Fitting a 5x7 on top of the bracket is tough. The inside of the speaker cover is about 5" and most 5x7s are 5 1/4" to 5 1/2" wide.