Here's some images of the support that I used for my front deck.  It's 1 1/2" aluminum angle.  The mounting plate for the seat does not mount to the angle, just the floor.  I've never had a problem with it being that way.  Hope this helps everyone who has asked for some images.

Here's a link to another framing job done by J.P. VanHoy in a Lowe 1436. CLICK HERE