Calling me a metalworker would be an insult to real metalworkers everywhere. That said, depending on the size of the piece of steel, it’s probably not worth your time to fill in then grind flat when local big box hardware stores sell common sizes of “welding steel” for relatively cheap and as the other poster wrote, McMaster-Carr has just about any size piece of angle you can think of.
Source: was a kid with no money scrounging in what was left of grandpa’s (tool maker) machine shop who is now an adult and now buys the parts I need rather than trying to make do