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My deck is an unusual shape. When installing lattice around the deck there is a gap where the lattice meets at the corners. Is there some kind of fastener to pull the lattice together?
Lattice, like any other thin materials, won't have a rigid edge, suitable for joints. You should try to always make your joints over some solid member, like a post, stud, etc. Your corners don't sound like there is anything like that present. You could make a post for the corner, running from the flooring to the underside of the rail, on the inside. Use a 2x2 or something even bigger. Cut it at the same angle that your lattice forms when the two pieces meet.
Now you can either nail or screw the lattice to it by anchoring the upright to the rail and the flooring. Now fasten the lattice to it. It will close the gap on the inside and give the lattice some rigidity.
If you're concerned about the outside of the joint, get some thin materials like 1/2" by 1 1/2" and fasten it over the joint on the outside of the joint. Use two pieces, one from each direction with the joint cut at the same angle as the post you made. Nail it on and you should be good to go.