Also, for practice, buy some tubing from Aircraft Spruce. If you buy some 1 inch and some 1 1/8 with a .058 wall thickness, the 1 1/8 tubes will just slip over the 1 inch. (Anything 1/8 inch bigger with that wall thickness will work) Miter them and braze them together as simulated lugs. Then cut them apart to see how well you flowed the filler in there. Practice a lot. Make sure your miters are tight. If you do that, you will be a bunch happier when you build a frame.