Inside is the best from a protection standpoint, of course. On the top of the tail box area is usually second. Side would be third choice for protection. If that is your only realistic option, though, I'd suggest putting it up as high as you can and on the left side. This side will get the least potential for impact from a main blade. Height should help protect it from ground contact or getting hit by a broken skid/strut. Hopefully someone with some TRex 250 experience will be able to comment more on if this is a good idea or not on that model heli.