I mounted a 1/4" thick aluminum angle to an old seat post I have, and used the cheap academy rod holders on my tin boat. Works great and no flex at all, although I hate the rod holders (they will be changed out soon)

The question about how to keep it from spinning? I tie off each side of it to the rear cleats on the boat so it has no where to go. I also use the seat post to tie off my drift sock too so it holds the boat centered down wind.

Originally I used the locking pin on the seat mount but later I just drilled through the seat mount and pedestal and stuck a bolt through.
This is the type of seat pedestal that does not spin at the base.

I have the cheap rod holders - looking to upgrade soon.