I want to fill the cavities in the top of a chinook finbox before installation. Is expanding foam ok and can I use the one-part stuff that comes in a spray can?

Can foam has all sorts of voids below the surface so once you start sanding flush you may find some big gaps. Pour foam is much more consistent and would be my suggestion.

What he said ^^^^

Just jam some styro foam in it.
More importantly, we need pics on the board build

After putting a new box in this one recently we are heading back to the shop. Hit a turtle . I'm happy my previous repair survived, but the box is broken inside. We will try a tuttle box this time - seems like powerboxes sometimes don't breakaway...