I have an existing deck that have posts extending from footers under the deck and all the way through for railings. The posts only go up 30” above the deck and I need to add 6” to add new railings. I saw a post about this but they added 12”. Is there a joint that would hold the 6” piece? Can I join it with 3” or is that not sturdy enough to support the top half of the New railing?