I don't think anybody actually GLUES it. The bottom of the Phantom is where most people mount it, many of us use outdoor double sided mounting tape and that works fine... I actually rigged a little harness using 5 zip ties since they're super cheap and I can just snip and re-make the harness anytime I want... the harness goes diagonal right next to the battery. I'll post a pic below you can barely see the harness but it applies more to your question about preferred location, there's really 3 ways to do it
1) pointed straight out the back, but offset to one side so that you don't block the battery access. This is probably least ideal because of antenna placement and your transmitter might be too "long"
2) instead of pointing straight out the back, set it at an angle. This actually helps letting you place your antenna well but be careful, you don't want the video Tx antenna right next to the Phantom's compass, the little silver block mounted to the leg in the lower right of my photo below. This is how I have mine setup
3) some people mount the video Tx at a right angle so the antenna points out the "side" of the Phantom, and that works fine if you prefer