Hotwire spacer for electric fence

In the endless war of gardener versus squirrel, I'm experimenting this year with an electric fence.

In my implementation, I've put a 3-feet-tall chicken wire fence around my tomato bed with an aluminum wire running just above the top of the fence. The aluminum wire is hot and the chicken wire is connected to the ground terminal of the fence charger. In theory, a squirrel that attempts to climb the fence will get a shock when it reaches the top wire.

To keep the wire isolated from the fence