A sodium lamp fixture, 10# or so.
I see two options: screw right into the tree (SS deck screws?), or big hose
clamps (or threaded-rod/plate equiv) around the tree.
Both can impact the tree, it seems. Mebbe a hoseclamp with springs, for
tree growth? That's a pita, tho.
Would be about 20 ft up, on the trunk, not a branch.
I would would proly put the weatherproof box to a long-ish board or alum
strip/bar, and attach that to the tree, for torque issues.
The trunk would be about 10"-12" in diameter at that height. How deep can
one screw into a tree without doing harm?