An 8"x12" surface plate would enable you to blue and scrape something longer if you do it across the diagonal.
Maybe one long edge of one of the various lattice style cast iron spirit levels. Here's an ebay example, though I'm not particularly suggesting this one. Perhaps a trip to a local scrapyard will show up a length of something cast Iron, from which you can cut, and machine a slice, before you strip the mill.
Without wishing to continuously direct people away from this fine forum, I've an album over on Flickr showing the various setups I used when fettling the various straight and V ways during the rather amateurish refurbishment of a Herbert grinder during the first covid lockdown.
I've not reproduced the album on here as there's the best part of 150 photos to resize, upload and caption.
Doing it on Flickr and also Facebook took me long enough as it was.
It may not be the textbook way of doing things, but I used what I had at the time, and some of the more inventive setups might give you some ideas to develop further, and better, to suit your own needs and available equipment.
Bill