You can mount it inside and its been done by others. If you have a mini iosd its because you are doing FPV. I've only seen one setup where the FPV and iosd are inside the phantom. The problem is that they get warm putting them inside prevents them from getting the right airflow. Flying in 15 degree weather helps but unless you live on the north pole, you'll eventually fly during the summer. I've seen a lot of extremely clean installs with the fpv/iosd on the outside. Also, having access to them without going through a load of trouble to get to them is helpful should you have an issue or need to update. Long story short, you can do it but there is no good reason to do it.