I think that most people will tell you that you shouldn't mount your antenna on the bottom of the plane, but sometimes there seems to be no other way. So when I have to mount it on the bottom, I run it through a short leingth of antenna tube ironed on to the tail of the plane. Then with the antenna through the tube, I secure it on the other side by sliding a piece of fuel line on the antenna and clamping it down with a zip tie. This method works very well, and if the antenna were to snag, then it would release way before any damage could be done.
Here's a pic

LT-40