when i made my chassis i made 2 jigs to keep it square. see pics, with the cut out you can tack weid all 4 sides. the jig must be dead square. clamping 2 at right angles you can get all the uprights dead on bass

Good idea Bass,

One thing I found really useful was those magnetic triangles. They're only about £1/£2 each (various sizes) from show's but they allow you adjust the angle of the tube until it's exactly perpendicular to the other tube it's about to be welded to.

James