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I’m nearly done a similar setup. Just be mindful that with only 3” gap on each side you don’t have a lot of play to mess with the bungies. It might work fine, but you could consider going slightly wider on the frame. For the bottom, consider shock cord like this, threaded through the bottom grommets: https://a.co/d/0ecolQlV For mine, I let the bottom edge of the screen (couple inches) lay on the ground and threaded shock cord through the grommets. Screwed the ends of the shock cord, pulled tight, into the subfloor. Then, put my flooring on that short lip of the screen so the turf runs seamlessly into the bottom of the screen. To be determined how well it holds tight, I haven’t gotten to hit into it yet as I’m mid install