I may have slightly over-engineered this, but as these brackets had rotted to the point where I had lost my outriggers altogether, I decided to produce new ones which will still be with us in another 50 years. Rather than using pressed steel, as was originally the case, I made it in three parts (from thicker material), welded together and to the outrigger. The weld is not pretty but it is very solid. Before welding, I drilled holes for bolting the backplate onto the chassis, and drilled the left side to allow me to spot weld it along the outrigger side, as you can see in the picture above.
The use of bolts will not make the outrigger bracket removeable but means that I won't have to weld to the chassis. I can't tell you why, but I feel I should avoid welds on the chassis itself, if at all possible. When the bolts (which also hold the reinforcement plates) go in finally, I'll secure them with a bit of weld so that they won't undo.
I shall do the other side tomorrow.