I'm far from being an expert at swarm-trapping, as I live in open countryside, and swarms out here are as rare as rocking-horse poo. But - I did once try, and nearly broke my neck in the process. But - by doing so I did figure out a method of attaching to a tree from a ladder (if you don't have a ladder stand-off) - assuming you still want to hang 'em high up - which I'm not recommending ...
First, pick your tree with the use of a ladder in mind - especially with regard to the softness of the ground. Then, up you go and hammer-in a BIG nail (say, 6") angled slightly downwards into the trunk of the tree, leaving about 4" exposed. This will enable you to then easily remove that nail afterwards.
Make your swarm-box the shape of a small suitcase, so that when holding it at the top of the ladder, your body is not being pushed away from the ladder, as would be the case if the box were (say) a cube. On the rear wall of the swarm box, attach a plank of wood vertically, so that 6-12" extend up above the box. Within that 6- 12", drill a large hole - around one inch diameter. It needs to be large, so that you can easily slot it over and onto the 6" nail - which will be above your head, and thus out of view.
Ok - now you have the swarm box in place, hanging from a nail - then you'll need to secure the box from swinging around in the wind. If the tree-trunk or branch is of relatively small diameter, just loop a ratchet strap around it. If it's too large to get a strap around, then use two more 6" nails to either side of the box, and attach a ratchet strap to those.
With regard to the box entrance, you need something simple which can be closed without any delay - such as a single round hole with a cork to match. What you don't want is something fiddly which might bring the bees out to investigate, with you at the top of that ladder.
All-in-all, it's much better to hang your bait boxes much nearer to the ground - at step-ladder height perhaps ? Whatever you choose - recommend you place a suitably-sized weight inside your box, and conduct a 'dummy-run' of both placement and recovery, 'cause once a colony is inside that box, it's then too late to experiment.
LJ