If you put your eruv good low in a tree, note that if it within 3 tefachim of the ground, it's as if it were on the ground itself. What if you put the eruv food in a basket that hangs from the tree? Will that hanging make a difference? Will it be valid? Can you move the food? Perhaps the eruv first must be in a makom patur, in its own defined domain. Also, how Rabbi Yehudah HaNasi uses the opinion of Rabbi Meir. Plus, when yom tov falls out on Friday, and you want an eruv techumin that will work for both for yom tov and Shabbat (since it's not an issue of cooking or carrying).