I was just thinking about ways to store a bunch of plate flowers (for gifts and also ones I might bring inside over winter)... they don't stack well, but they'd hang just fine on studs in the (unfinished) basement. With glass bottles already on the backs, picture hangers won't work, but I'm thinking I could use big long nails... hammer them into the stud some, then give them a couple of whacks to bend the nail upward... the nail could then go up inside the bottle the same way as a stake or rebar would in the garden.
Lovemygarden, this idea might work for your fence, maybe using long-shank screws and then bending them upward.
As for placement on your fence, I think you might need as many as 6 to start... place them near the tops of the posts (as you've done in the photos), and also put a plate between the 2 posts on the center rail... sort of a zig zag effect, filling in the space without putting them in a single straight line along the top rail. If you don't want to put them on the gate posts (to avoid having them banged into), they could go on the top rail near the gate posts instead.