Creating a custom serverless resource for subscribing to SNS topics in another region

serverless.yml

# Welcome to Serverless!
#
# Happy Coding!

service: cross-region-sns-subscriber

# Keep environment specific configurations in separate files
custom: ${file(config/${env:STAGE}.json)}

provider:
  name: aws
  runtime: nodejs6.10

  stage: ${env:STAGE}
  region: ${self:custom.environment.region}
  role: arn:aws:iam::${self:custom.environment.account}:role/${self:custom.environment.lambdaRoleName}

  environment:
    STAGE: ${self:provider.stage}

functions:
  processSns:
    handler: handler.sns # entrypoint to your lambda
    memorySize: 512
    timeout: 30
    events:
      - sns: ${self:custom.sns.arn}

  # Below function is used by cloudformation to create the sns subscription
  topicSubscription:
    handler: topic.subscription
    memorySize: 256
    timeout: 30

resources:
  Resources:
    # This resource has to match the name of the generated SNS subscription resource
    # in order for it to override the implementation
    ProcessSnsSnsSubscriptionMyTopic: 
      Type: Custom::TopicSubscription
      DependsOn:
        - TopicSubscriptionLambdaFunction
      Properties:
        ServiceToken:
          Fn::GetAtt:
            - TopicSubscriptionLambdaFunction
            - Arn
        TopicArn: ${self:custom.sns.arn}
        Endpoint:
          Fn::GetAtt:
            - ProcessSnsLambdaFunction
            - Arn
        Protocol: lambda

topic.js

'use strict';

/*
  This was adapted from https://gist.github.com/Nath-P/899e5e021b9a19b3e601ccae083606fb
*/

const response = require('cfn-response');
const aws = require('aws-sdk');

module.exports.subscription = (event, context) => {
  console.log('REQUEST RECEIVED:\n', JSON.stringify(event));
  let responseData = {};

  if (event.RequestType === 'Delete') {
    let subscriptionArn = event.PhysicalResourceId;
    let region = subscriptionArn.split(':')[3];

    aws.config.update({region: region});

    let sns = new aws.SNS();
    sns.unsubscribe({SubscriptionArn: subscriptionArn}, function (err, data) {
      if (err) {
        responseData = {Error: 'Failed to unsubscribe from SNS Topic'};
        response.send(event, context, response.FAILED, responseData);
      } else {
        response.send(event, context, response.SUCCESS, data, data.SubscriptionArn);
      }
    });
  }
  else if (event.RequestType === 'Create' || event.RequestType === 'Update') {
    let topicArn = event.ResourceProperties.TopicArn;
    let endpoint = event.ResourceProperties.Endpoint;
    let protocol = event.ResourceProperties.Protocol;
    let region = topicArn.split(':')[3];

    if (topicArn && endpoint && protocol) {
      aws.config.update({region: region});

      let sns = new aws.SNS();
      sns.subscribe({
        TopicArn: topicArn,
        Endpoint: endpoint,
        Protocol: protocol
      }, function (err, data) {
        if (err) {
          responseData = {Error: 'Failed to subscribe to SNS Topic'};
          console.log(responseData.Error + ':\n', err);
          response.send(event, context, response.FAILED, responseData);
        } else {
          response.send(event, context, response.SUCCESS, data, data.SubscriptionArn);
        }
      });
    } else {
      responseData = {Error: 'Missing one of required arguments'};
      console.log(responseData.Error);
      response.send(event, context, response.FAILED, responseData);
    }
  }
};

Is there an example of ${file(config/${env:STAGE}.json)}?