I use both the iPowerUS 220mAh 9.6V and Powerex 200 mAh 9.6V in the HeadRoom microDAC + MicroAmp. With the powerex, just shove them in and let the flap get pushed aside. For the iPowerUS, you need to lift the rubber flap, insert the battery, and let the flap hang over the back because the battery is shorter.
I wouldn't buy the iPower batteries again. They seem really cheap compared with the Powerex. Maybe I got a bad batch, but several of them rattle, and two of them are just plain subpar, getting only half the playtime of the other four (I bought six). The good ones do run a bit longer than the Powerex. This is just anecdotal evidence (maybe the worst kind) but you might want to be careful if you try them and make sure you can (and would care to) return one or two of a batch. I'd rather spend the extra buck and deal with the 10% or so less playtime.
One other consideration is that rubber flap. Occassionally, the batteries don't catch well, and I have to pull them and reinstall. This doesn't happen with the rubber flap down, so that's a plus for any shorter battery.