Just lean the lattice up against the wall at an angle, then just push the tops thru holes. Onions stay out, leaves stay in, no rain goes thru the leaves into the onions.
The water down the "leaves" is what causes the internal layers (or rings) of the onions to rot.
It was that one year you get with the most beautiful onions bigger than your fists. Happens like one in 10 around here. So much rain. I had a whole row of white, a row of yellow, and what was supposed to be a 1/2 row of red (it was slightly more red than a pink color... had streaks). LOL
I don't plant anymore....... BUT, what I learned by accident was that the latticework that goes around the bottom of my house, which I had a bunch of old sections in the barn was unbelievable for drying onions. As long as the rain doesn't go down thru the top the onion didn't rot, if it rains on the bottom facing up I had no damage. I just dropped those onions down thru the lattice.
Fast easy and no rotten rings inside the onions.
I know we are later down the calendar here, but down south you leave them all year on lattice? in shade of course, ya know can't do that up here, just put on door screen in basement off floor and need to put a fan down there which I never did before and they would not last very long. Same with taters.