spoke with a mate (fabricator) about this... he has a totally different view....
Draw a line where you want the join...
now the next part is down to skill... make a template of the curvature of the panel along the line...
he said at his place they have one of those things that you use to mark fancy shapes for cuting tiles but its about 2m long
they would then cut that profile twice once for the bottom once for the top edges...
then shape it for the upper panel... and the lower, but the lower makigng sure the lip allowed for a seal to be applied...
the shapes being a sort of L
the lower lip longer so that there is space for the seal...
he said they would gut the line with a 2-3 mm disk thus allowing a small gap....
and weld the lips on to the panel edges
like this...
it should be welded at the ends to the sides of the panel and he would add some triangular strengtherners between the lips and the panels
if tig welded on the inside it should be possible to grind the edges round so it looks like it was pressed...