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I wish you'd asked about Janczak instead!

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So we can leave out Shedlock, Shedlak and Sydlak as misspellings of clerks unfamiliar with Polish. That leaves Siedlarczyk, Siedlak and Szydlak, all known Polish surnames, none really common.
I would treat Siedlar as a variant of the more common Siedlarz, and Siedlarczyk is almost certainly a diminutive form of Siedlarz. The endings -arz and -arczyk are usually found on occupational names, e.g., Bednarz ("cooper"), Bednarczyk, Tokarz ("turner") Tokarczyk. However, I can find no explanation of the names Siedlarz/-arczyk, as occupations or as any other surname source. The nearest word I can find is siedlisko, "home". Associating Siedlak (rightly or wrongly) with the Czech Sedlák, "farmer", I entertain the notion that it might mean something like "homesteader", and Siedlarz/-arczyk alternative versions of the same.
Unless, of course, these Sied- names turn out to be alternative spellings of Szyd- names. Pronunciation of the two spellings is not very different.
Now Szydlak: this too I have not found in dictionaries. The nearest word is szyd£o (with a barred L) meaning "awl, bradawl, bodkin". There are some surnames based on the word szyd£o, e.g., Szydlowski, that are said to be metonymics indicating descent from a shoemaker. Perhaps, then, Szydlak has the same meaning.
I don't know if any of the above is of the slightest help, better than nothing I suppose.