If not you can get rounded edge slotted bases from eBay for next to nothing. I got a pack of 30mm ones recently so I could base the void spiders. You'd have to greenstuff it to fill in the base as it's a little recessed, but that wouldn't take much effort really.
I was missing an xxl base for the Ancient One, so when I ordered something out of their web store I put a comment on the order saying I was missing a base. When my order arrived the base was included with a hand written note on the packing slip that said something nice to the effect of happy gaming or something like that. I would just shoot them an email. I’m sure they’ll take care of you.
I had this issue when I was building mine as well. Was missing a 40mm for my last Tred Grenadier, then I realized I had and extra 60mm. Turns out I put my Swamp Raptor on a 40mm base, so I just popped him off of it and all was good.