1) draw by hand some glyphs, so you have an objective piece of paper of what you want
2) search on Internet for a font that looks like  what you (think) to want
3) starting from this font,  make some modifications of glyphs that you have drawn
with fontforge or metapost
With fontforge, you can even make countours from  images  , and with a bit of hack from MetaFont too.

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luigi


Don't forget about other sources of inspiration. Frederic Goudy for instance made illicite rubbings in Louvre and studied  Roman inscriptions on columns and other monuments, as many others did :)

Best of luck!

Piotr

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http://okle.pl