We use 2x2' pieces of hardware cloth with 1/2 or 1/4" openings (2 or
4-mesh). We usually buy a 50' roll and cut into pieces, but you can buy
smaller sections. It does a good job deterring voles and mice, and
rabbits if there's no snow. We can have enough snow that rabbits could
easily reach the tree during the winter. Luckily, our planting is out
in the open and the rabbits stay in the woods where the birds of prey
can't get at them as easily. The 2-mesh stuff is more rigid than the
4-mesh and we can set it up a little easier. It can be tough on hands,
and when vegetation grows up into the guard it can be a big chore to
remove. I've heard standard aluminum window screen works well enough
and is probably cheaper and easier to handle. I'm going to try putting
some pea gravel around the base of the tree to keep down the
vegetation. I wouldn't use flashing as you want definitely want airflow
around the trunk. . . . my two cents.