I then cut some 3mm plate for 2x CP1 and 2x CP2, and welded them on. I then cut some 2mm plate to use for the 8 blank ends for the exposed RHS ends. Thought id post up a little tip (or maybe its ovbious!) that was shown to me a good few years ago, but saves loosing the cut end piece inside the tube. Ive also done this with bolts in the past (although a bit wastefull!). Normally at work im capping 50x50x2.5mm RHS, which is easier.
Tack-weld a welding rod to the cap, and offer it up to the end of the box section.
Tack weld the capping plate to the box section.
Then snap off the welding rod, weld up the seams.
Then buff flat, and jobs a good 'un!