You can buy Permatex Prussian Blue spotting fluid, over the counter, at most Napa auto parts stores. It is a non-drying fluid used for checking bearings and such. It is also relatively cheap at about $5 per tube.
It also KEEPS well.. easily over 40 years in that handy tube. Which is good. Because a tiny amount goes a looong way.. annnnd we don't actually use it very
often..for "modern" engine work. Which can include checking flanges and anything else that needs to mate-up well - not just bearing shell fits.
And yes, it
can be substitiuted for Dykem Hi-Spot, but still.. Having BOTH, I tend to prefer the Dykem. Or Stuart's Micrometer, mixed to suit me. Both have colours other than just blue. Cromwells will post Stuart's from the UK to USA.
Cost of any of them is trivial for infrequent use.
Or compared to the (time) value of the labour involved.
Even so, put your "most important money" into decent edge sustenance on your scraper(s), and trustworthy metrology.
With bad reference surfaces or poor tools, you will simply be working hard at losing a poorly planned argument .... with your own sweaty best-efforts.. but gone astray!
Hand scraping for fit is only about 5% about HOW to scrape anyway.
The other 95% is WHERE and by how MUCH.. so it comes good as to
fit and EARLY in the process.
Not just "looks a pretty pattern" after too many goes and far more metal removal than need be to JF work well.