Rear bumper/aerodynamics question

[67carry-all]

I'm replacing the rear bumper on my truck and it occurred to me that not only is it a bumper, but also a giant air catcher that is the exact opposite of aerodynamically efficient (and yes, I know the overall truck is a giant box). So I was wondering, has anyone "filled-in" some of the empty inside space or done anything else to make the bumper less of an air catcher, and if so, did it seem to make much difference, especially with mpg at speed?