Depends how you wanna approach it! Like someone said, they can be thematic to other existing symbols in the world (other astrological symbols, alchemist symbols, etc.) There's also the Hiveswap Extended Zodiac symbols that could cut a lot of the work and you can just pick one from there, OR if you'd like to get more original with it -- if you observe the Extended Zodiac symbols you can observe the symbols across each blood caste have a pattern to them and base something off of those patterns if you wanna be a little more homestuck canon-adjacent!

If you wanna get even MORE creative with it, i think noting that the design language of the troll symbols are generally very simple, line-based designs is something good to note and you can try to design something with that line of thought! They don't really have graphic iconography (say, compare to the kids' shirts)