I, too, built my rig to hold a laptop. I've got the X-Air program running on an old Dell B210 running Ubuntu, and connected to my XR18 using ethernet (via a Netgear dual-band wired/wireless router). I've found that I can run the default music player in the background on the laptop, and am sending that audio to the mixer over the USB returns. I've got a bag of el-cheapo Android tablets for remote mixing (a 10.1" Digiland for me, and several 7" Digilands for performers for monitor mixing).
I've uploaded a crappy picture of my rack, taken before I put the USB cable in. The rack is one of those SKB 4-space "studio flyer" racks, and there's plenty of space in the back for a Gator bag full of tablets (I'm using the Gator bag that's a perfect fit for the mixer itself). One thing you can't really see in the picture is the 7-port powered USB hub behind the router, which I use for charging tablets during breaks.
So far, this is working great. I did a large outdoor venue a couple weeks ago, and was able to roam about 100' from the stage before I started to experience any control issues. I've a small club gig tonight, which I anticipate will go very smoothly.
I've got to say that, as much as I liked my Mackie DL1608, I also don't miss it now that it's gone to its new owner.