Geezer,
I will let Martin speak for himself, but I would not leave them to dew and rain. Also I don't hang the onions by the leaves. The smaller tops dry and bulb often falls off the string. You could set up shelves of hardware cloth, say 1/2" and lay them on the shelves to cure. I just spread them out on cardboard but then I have a lot of floor space.
You can hang the garlic in tiers, ie up high and then lower and lower. about 12 to a bunch for the smaller. I do 8 for elephants.

I indeed have left them out through a rain with no problem in the past but that is not my normal procedure. That tree-hanging was something done mostly with the shallots. If the garlic got sprinkles, it wasn't always intentional! After a brief drying, I've got 4x15 plastic mesh bags which are hung from the rafters of the garden shed. I can only fit about 15 normal garlic bulbs in each so they easily allow air to circulate through them.

Tree-curing for shallots is out now. Apple tree got a major pruning this year and only the deer-hanging branch was left!

Martin