What is the best way or how do you suggest i secure plywood to a aluminum bench seat?  I want to put a post on the back bench and i know the aluminum alone will not hold it in

If you can get to the other side of the aluminum, you can just stainless steel bolt the wood onto it.  If not then i think you can use toggle bolts.

We just go directly to the seat with an aggressive thread wood screw w/ 3m 5200 on the threads so it won't back out.

You can use something called a 'nut-sert'. It attaches similarly to a rivet but it's threaded on the inside so you can bolt the board to the aluminum. Similar anchors are used to attach side mirrors to trucks, etc. They install as simply as drilling the hole, dropping them in and running the bolt down into them. Tightening the bolt compresses the nut-sert so it squeezes around the hole and locks itself in place.