16d and 20d galvanized nails, with a few pole barn spikes and deck screws thrown in for good measure for me. I don't like to depend on a single fastener! So if I were strapping something on, especially at 24 ft, I would want two straps. And make sure the squirrels have not taking a shine to your straps, even if multiple ones are used!

Things like this terrify me, I ascend and descend with my harness on and attached to the tree at all times, sure I can only go a couple feet at a time and it takes me a lot longer than most to go up a tree, but I feel much better about it in the long run.