If you like the custom report you've created and saved, you can clone it to your library so you can use it for any future event. This will save time with report creation.
On the report you want, select the "clone to library" icon in the Actions column. Refresh the page.
When you want to use this specific report for a future event, click 'Create Report', then "Duplicate from Library". Click on your cloned report and it will appear in your list.
Now you can export this report from Reports list.