Gary that frame is for a cat. I would not build a cat frame for a raft. Especially not a RMR. I mean come on, you said that boat would only last 5 years. Any ways it is 3/4 steel. It is a Frankenstein frame. The seat trusses are square tubing and the rest is round. This will be the one of the lightest ones l have built. Less than 40 lbs.
The steel costs about 125. I have 3 days in them once they are done. This is my 4 one. I have changed the design every time. The first one was all square tubing. It is on my purple sotar. Then l figured out how to make my hydraulic bender do nice corners out of round tubing.
If you have some skills you could build one. Steel ttubing is the way to go for me.