I use several methods for rounding edges.
One is to place a sheet on a padded bench pin and hold down with one
hand while running a fine file across one edge and then, turn it over
and do the other.
I use my GRS ring clamp to hold some pieces. Because it swivels, I
can turn it to the most advantageous direction and then use my files,,
or in some cases, a sanding stick.
When the pieces are small and would be bent or damaged by other
means, I use my benchtop vice, with leather covered copper jaws. The
wide jaws make it possible to file on long, thin, short pieces
If I had to do a lot of large sheets, I would make a jig.. putting
two pieces of wood together, leaving a space just a bit wider than
the thickness of the metal,, and wide enough to accommodate the
width. and height. Would then be able to drop the metal in it,, and
run the file or sanding stick over it, as the block held it stable.
It is sooo interesting to find how many ways there are to do any one
thing.. It makes for a lifetime of learning.. and fun.