Ordinarily these are stamped out of steel, or cast in like 356 aluminum or thereabouts. Never seen one welded, but shrinkage could be a problem when you try to get it seated on the engine block.
If you had a casting, you could easily machine it nice and flat to get a good mating surface. Might look real nice in bronze all polished up.
If you cast it - you can make any design you want, art deco, hot chick, logo, lettering, whatever.
You could probably come up with a VERY slick looking valve cover by welding pieces together to appear like a heat exchanger, but I'd keep the base a little thicker so that you can polish it flat in case you get any warpage.