I got a genuine Alpine cage off Ebay when I had the same problem... only to find Alpine had changed the way their stereos clip in so it still didn't work.

All I did in the end was removed the small piece of trim to the left of the HU hole, and drove a single self-tapping wood screw in (through the plastic) far enough to push the side of the cage into the HU and hold it there. Works suprisingly well, and with the trim back on it looks completely normal

Has the added bonus of make my HU really easy to remove if I need to get to the back of it, pop off trim, undo screw a few turns and out she slides...
