The tree will cause the thermometer to read a little low in the day, and warm at night. Also the humidity will be higher under a tree all the time compared to an open skyline, which is where you are technically supposed to mount an air sensor. We all do the best we can, and if this is what you have, then you do what you have to. It is definitely better than against the building.
The shield looks like a really good build to me. I made one myself also, but the plastic was brittle like yours, and the last hurricane beat it up some, but I still use it.