For RDS Custom for SQL Server, where can I find a CloudFormation script or template with prerequisites?

You can find it here, along with a tutorial: https://aws.amazon.com/blogs/database/get-started-with-amazon-rds-custom-for-sql-server-using-an-aws-cloudformation-template-network-setup/

Please note that the script above does not set up absolutely everything you need to connect via Remote Desktop. For remote desktop and/or public access, please consider that you may need to set up additional networking. Also, see the official docs on setting up Remote Desktop (RDP) here which have additional necessary steps: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/custom-creating-sqlserver.html#custom-creating-sqlserver.rdp