Okay, for a barn, if you have a really tall barn a really good place to hang the trap is up in the peak of the barn. The peak of the barn is a hard place to get to, and you don’t want to be going up there to change out bottles. This is why you want to use the 2-liter bottle.
Screw the 2-liter bottle on the trap and be sure to add a hole in the bottom so the bottle doesn’t hold any moisture; you don’t want to climb up there if a little rain gets in it. Then, hang or screw the trap to the peak of your barn, on the sunny side. Then watch that 2-liter bottle fill up with bees! I have a friend who has a trap just like this with 2-liter bottles full to the top, it has taken a couple of years, but he has literally caught thousands of carpenter bees.