Hi. I recently bought an unfinished cabin on the Olympic Peninsula in Washington state. I am planning to make the crawlspace sealed and conditioned, and so intending to insulate the interior foundation walls. For this, foam board seems to be the preferred option.

However, the cabin has a “Fastfoot” foundation, which is basically poured in a big bag and so it has a bulbous shape at the base. I’m attaching a photo from the exterior, but it has the same shape on the interior side of the crawl space and I’m not sure how one could get foam board to work. I’d like to avoid spray foam if I can but is there an alternative technique and/ or product that would work for this application? Could I sort of “box” it in with foam board and fill the gap with canned spray foam? 

I’ll need to insulate from the base of the foundation around the bulbous bit and up the vertical stretch at the top, which is anything but a flat, linear wall unfortunately. Hopefully this makes sense along with the photo, I kind of  scribbled in red the countour of what I need to insulate (of course mirrored on the interior side), but let me know if I can clarify or provide more information. And thanks in advance for any advice.