I worked in graphic arts for 35 years, our favorite straightedges were
hardened steel, not neccessarily heavy but usually backed with
something to keep them from slipping, sometimes only a strip of masking
tape. Thicker materials for backing would raise the straightedge too
high, allowing your blade or whatever to undercut it.
If you're looking for machined straight edges cheap, check with print
shops and ad agencies, or watch for auctions from them. The edges of
their light tables have a removable, adjustable steel bar, 1-3" x 3/8"
x size of table side that is precisely machined. Probably not Starret
standards straight, like for setting up a jointer, but plenty accurate
for what you want. They also make a good router guide for jointing
boards without use of a jointer.
There are many abandoned light tables all over the country, from a time
when most layout work was done by hand. Since the bars are usually
chromed, rust is not a problem.