Filter Search By Domain
Instead of creating a Frase document crawling the top 20 results from Google for a search query, you could filter by specific domains (instead of general top 20 results) when searching the query.
When the document creation window is opened, click the "Advanced Settings" text at the bottom. Then select "results from a specific domain".
After typing the first domain, click the + icon on the right to add another field for another domain to add.
You can filter by 2 domains without the Pro add-on, but with the Pro add-on included, you'd be able to filter by 5 domains.
After adding the domains, make sure the "target search query" field at the top is entered to create the document, and Frase will look for results only from those domains when pulling sources via Google.