I have made a lot of cross, seat and other bars. Your on the right track. For a gear boat configuration the seat bar is a waste of space so turning the cooler or drybox into a seat is the way to go.
All you need is a couple of lo-pros. If your frame has the previous style it will be easy to find some of them around. Otherwise just buy a couple from NRS or any dealer that has them. buy the length of 1 1/4" schedule 40 6061 from a local dealer and voile
You can buy anodized or powder coat the bar but at that point you will probably be close to the NRS price.
One thing to remember is that PVC doesn't like raw aluminum it will leave nasty black marks on your rubber. Another option is to buy some clear gorilla tape and adhere it to the bottom of the bar where it touches the boat. I do that on all of my frames in order to keep the boat cleaner.
another option is to watch your local craigslist ads and pick up a used NRS bar that is as long or longer than you need.
You can use other fittings like clamp or speed rail but in the long run you will be happier if all of your fittings match. IMO