The AWS Serverless Application Model (SAM) now supports REGIONAL API Gateways, just add EndpointConfiguration: ‘REGIONAL’ to your AWS::Serverless::Api object:

ApiGateway:
  Type: AWS::Serverless::Api
    Properties:
      DefinitionUri: apigateway.yaml
      StageName: api
      EndpointConfiguration: 'REGIONAL'

This is currently an undocumented feature but in my testing is works fine.