I have tried a lot of things, and this 4 foot fencing is the one I use now and have had zero deer damage since using it.
Deer Fencing
I cut it into 6 foot sections and put it in a circle around the tree. This length stands up easily by itself and is resistant to falling over, especially once mulch is added. It is a little pricey but only comes out to about $7 a tree which is worth it in my opinion to protect the investment of buying the tree.
Smaller sections can be used but require staking and other effort and can still fall over in my experience.
Here's a picture from above looking down.