If you're theme is a cemetery, why not fence it in and use a pillar to support the screen?
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Depending on where the entrance to your house is, you could even create a mock entryway to the graveyard and make a gateway with pillars supporting the gate tall enough so that one can act as your base to string the cable. The other is purely decorative. By having a gate, you break up the imagery that you’re showing and that adds to the ethereal effect you’re hoping for.
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By creating a fence around your property, you also keep people from inadvertently deciding to walk across your yard and right though the mostly invisible cloth you're going to use to display your images. https://www.instructables.com/id/How-to-Build-Halloween-Cemetery-Entrance-Pillars-G/
is a good place to start, or just Google "making Halloween cemetery pillars" and go to town.
On the back side of the pillars, you can attach eye-bolts or your favorite way of attaching the cables to the bottom and top. It's facing away from the street, so making it unobtrusive should be fairly easy.
If you're not up to committing to an entire fence, or just the gateway, try an obelisk framed in wood, properly weighted, and staked to the yard so that it won't fall over.
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Frankie’s Girl made a great one out of all the stuff you shouldn’t use, but the design elements are there so you might give her post a look. https://www.halloweenforum.com/halloween-props/69352-obelisk-under-6-a.html
. Once constructed, use the same approach as you would the pillars to attach the screen.
Both approaches leave your electrical box completely unobstructed, which is really a good idea. Sure, put a box over it Halloween night if you like, but don’t rely on it to provide any kind of support. You don’t want to hear what others will say about that if you have an HOA.
Good luck. and have fun.