Choose Fixed Price for project model and Roles for Budget model.
Zero out the project rates by clicking on the Project Rates tab, changing the Rate card to "internal" and clicking on the arrow to update these.
Click on the Other Expenses tab and add monthly retainer with the correct date and amount.
Use the clone to add as many as needed - making sure to update the dates for each period.
Add the Other expenses for the year (or specifc time period) on the Other Expenses Budget.
Copy it into the overall budget amount and click Save.
Now, the retainer will be recognized regardless of the work completed.