People used to build frames out of wood. For a cheap raft anything that you can put together (without sharp edges or severely protruding sections) should work. I'm not familiar with fencing components, but I do know if you put together a decent size order you can get a discount from Hollander. Fencing steel isn't going to be as strong, and there is a Buzzard (who isn't posting much lately) who uses that steel and just puts wood plugs in the ends to add strength where the connection forces are.
But why not get speedrail, you can reuse those components when you build a frame. My frame is a hybrid of NRS pipe, NRS fittings, and a lot of speedrail. NRS pipe is easy to work with, and it's fun designing and building your own frame.
I am a river, babe - I've got plenty of time, I don't know where I'm going, I'm just following the lines..... - "We are water" by Shaye