The solution was to use a download manager that allows continuation of downloads. My preference is aria2.
I used chrome, so that's the process I'll describe.
- Open up the Developer Tools, then click the download link.
- View the Network tab and identify the request, likely it will come from a googleapis.com subdomain.
- Right click the request and Copy request location.
- Click on the Headers tab and in the Request Headers section, locate the line beginning with Cookie:.
Combine all of the information you now have into a download command with aria2 like so:
aria2c -o export.mbox -c --header="Cookie: AUTH_e2e0q...etc" long-request-url-goes-here
That should do it, and if all goes well it will download completely.