Try S3 for static content, then fallback to EC2 using Nginx as reverse proxy

nginx_try_s3_fallback_ec2.conf

location ~* ^/static/(.*) {
  set $s3_bucket        'your_bucket.s3.amazonaws.com';
  set $url_full         '$1';

  # Not sure if I need the following options or not, need to look into it.
  proxy_http_version     1.1;
  proxy_set_header       Host $s3_bucket;
  proxy_set_header       Authorization '';
  proxy_hide_header      x-amz-id-2;
  proxy_hide_header      x-amz-request-id;
  proxy_hide_header      Set-Cookie;
  proxy_ignore_headers   "Set-Cookie";
  proxy_buffering        off;

  proxy_pass http://$s3_bucket/$url_full;

  # Proxy to ec2 as fallback
  proxy_intercept_errors on;
  recursive_error_pages on;
  error_page 404 = @dynamic_ec2;

  # use aws dns resolver for potential speed
  resolver 172.16.0.23 valid=300s;
  resolver_timeout 10s;

}

location @dynamic_ec2{
  set $ec2_host 'your_bucket.s3.amazonaws.com';
  proxy_pass http://$ec2_host$uri;
}

# source: http://linuxplayer.org/2013/06/nginx-try-files-on-multiple-named-location-or-server
# source: https://coderwall.com/p/rlguog