of an "unknown egg sacs" section, and I had made one but it was deleted. We've accumulated quite a few unknown egg sacs (including many examples of those pebble-covered ones in Washington) and I think it would make sense to put them together.

Even with it being as big as it is?
I don't know, it makes me think of a bigger type spider like a funnel web spider or something (I know it's not a funnel web).

I wish the books would include more measurements in their descriptions, but the egg sacs of crab spiders are described as "large," and many of them are definitely disc-shaped like this.
Here's one that's sort of similar:

...Or it could be from a philodromid crab spider, or I could be completely wrong, but crab spider was the first thing that came to my mind. What other big spiders might it be (just thinking out loud here)? If we can rule out agelenids, orbweavers, wolf spiders, fishing spiders, brown recluses (the size and shape are right for that, but not the texture)...

How about this one?

The size is exactly right, and the shape looks good. Yours has a neater margin, maybe, but I think it's a pretty good match. I'm trying to (mostly) finish up my spider egg sac chapter this week, and I'll let you know if I come up with any other plausible matches, but so far this is it.

Cool! I would never have thought an egg sac that big would belong to a crab spider.