Looks like you have the same issue I had 2 years ago (almost to the exact day!). See here:
Lens hood part number for Sigma 70-200/2.8 APO EX? - PentaxForums.com
I contacted Sigma and was told that the hood to fit the older APO EX lens was no longer available. I also bought a Sigma 70-200 hood from KEH - they couldn't tell me which version of the lens it was for, but for the price I took a gamble (and lost - anyone want to buy a hood for the later version?
). The hood for the newer versions is slightly larger and does not attach securely to my lens. A local camera store had this one -
https://www.amazon.com/Promaster-77mm-Universal-Lens-Hood/dp/B0039PIRGM?tag=pentaxforums-20& - but at the time I was still on the hunt for an OEM hood. I ended up with an unbranded, screw-in metal hood that reverses for storage, and it's been fine. It provides enough protection for the front element, and while it's not as deep as the OEM petal hood, I think it still helps with flare/contrast/etc without causing any vignetting in the corners.