There is nothing worse than wobbly railings. They should be sound and sturdy.
You do have quite a challenge, but it is possible. I would get a post hole digger and dig holes 8 feet apart and as close to the existing deck as you can. Sink 4 x 4 in the hole and concrete in.
Once the posts are in you can make the fence cantilever over the deck itself using many techniques.
This is the best method that I can think of.
You could also carriage bolt in a 4 x 4 to the structure of the deck if the structure is even with the decking.
If it is not, could you knotch or cut out some deck boards and dig a hole for 4 x 4 posts?
Study it out and don't rush into it, because there is a way.
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