Thanks. Informational, but he didn't resolve the problem. I thought he did when he described the notch at 2:11, but I compared both of mine, and they both have the notch cut in it.
This is what it looks like if I try to put it in:
There's still some space it needs to go before it's fully in there.
And the larger view:
I think this little ledge here might be the culprit: