Usually when I create a glyph for the script, I turn to my sources on Finnish etymology, lexical and morphological. Then I draw them as pictures and abstract them; but sometimes this just doesn’t suit me, like when I do not know the etymology, or the morpheme is just a slight sound-change away from another morpheme that has nothing to do with the new one’s concept.
The Finnish adverbialising morphemes -ten, -tten, -tta and -sti all have in common a t-element. Some of these are ancient Uralic adverbializers, like -ten, and one is just the lative -s + ta: the exact same as the Finnish elative, derived from it, in fact (according to Häkkinen), but this seems meh to me. This is when I decide to ignore the reality of etymology behind it, and reanalyse the morphemes to suit me, as I suspect might happen with a real writing system. The script is not meant to emulate etymology, it is meant to represent speech: I do not need to chain myself to it. Thus, all -ten, -tten, -tta and -sti all contain material from the same glyph, one where two human figures, divided by a vertical line, mirror each other as they wield a stick. The glyph -TEN1 is basically this, with historical abstraction. -TTEN is a further simplification of that original glyph, and -tta and -sti (from *k-ta and -s-ta) have the latives (etymologically correct) -k and -s, plus another, different looking, simplified glyph based on the ancient TEN1 glyph. This way, I get a bunch of glyphs without needing to make up separate etymologies for them: somewhat realistic and practical.