I made up a few welding jigs - 2 right angle jigs, and one variable angle jig.
the right angle jigs are pretty self evident in the photos - 2 pieces of angle iron welded to some strip steel whilst maintaining the right angle. The stock being welded can be clamped inside or outside of the angle iron, and the gap is sized so I can still access 2"SHS pipe for a complete weld. The design for this jig is adapted from one I spotted on instructables.
The variable angle jig is simply a variation where the strip steel is replaced with a bolt and wingnut. To add rigidity, and to allow "preset angles" another brace is used to hold the arms at the correct angle.
When I first made it, I figured that a four sided plate, with holes in the vertices would make a simple "preset" for 30, 45, 60 and 90 degrees... but mulling over it later I came to the conclusion that a plate weighs considerably more than 4 narrow strips, and would reduce the chance of the plate getting in the way of things being worked on.