I think fabricated valve covers all depend on who builds them. Some guys seem to know how to do it while others just build junk. The good ones cost a fair amount. To do it correctly a person probably needs to invest in some proper jigs as well as some sort of tempering oven to relax all of the welding stress. Nice thing about billet parts is that 6061 aluminum tends to be very stable. Pretty rare for the billet stuff to warp over time.