Is there a rule for repeated letters when creating acronyms/initialisms/abbreviations? I'm writing my master thesis and I've created an initialism that has the letter S repeated three times in a row (the initialism is OSSSP and stands for Optimization Search Space Scope Parameters). In my opinion, the constant repetition of letters in initialisms, abbreviations, and acronyms makes them look ugly, but I couldn't find a well-accepted rule for what to do in such scenarios.
Turn my initialism in something like 'OS3P' is an option, but it seems to be naive and informal. If I knew how to do it for this specific case, choosing synonyms to replace the original expression would be an option too. Any other thoughts on this?
Turn my initialism in something like 'OS3P' is an option, but it seems to be naive and informal.

There's no rule against such kind of initialisms, and I wouldn't consider it naive either. Actually, there are standards designated in similar ways, e.g. the I2C and I2S data busses, also spelled I2C and I2S.