I would use one of the aluminum rail mounting systems, but I have seen them on a 2x6 running across the stern attached to the railing with ubolts. It can be simple and cheap or fancy and expensive.
Something like this out of a box, or homemade.
http://www.bigpapasportfishing.com/t...olling_bar.htm
2006 Forester 19 Fish (new deck and carpet, Pontoonstuff interior, 2019)
1996 Mercury 50 ELPT4S
1983 Sea Nymph FM171 Striper (complete rebuild from hull up, 2014)
1985 Johnson 70 J70ELCO
Raystown Country, PA