Scraping master

I am looking for a chunk of cast iron to fabricate a scraping master for the
V ways in a surface grinder. Preferable dimensions are about 4"-6" square
and 36" long. Hollow box shape is preferred over solid as I do not wish to
handle a solid chunk of iron. Can anybody think of a source for such a
thing? (Cast mailbox posts, etc.) Steel will work but it is a pain to
scrape in.

Doug