I have two of these rails which are easily dismantled and fit in holes drilled in ground,
holes not same so cant mix them up
made the verticals first then shimmed up and welded the top with a bubble level
when both are assembled you can stand back and sight both rails perfectly level
them magnet square is a waste of time, get a good carpenters square, double check it both ways and make your own
square jigs like what I have here and use the squeeze clamps instead.
It better to clean and prime the steel first, then cut to suit another day, less headache to prime after welding.
plus you will likely clean it much better when they are just in lengths