The template I use is very simple and relies on uploading a package bundle as a TXZ file that contains a setup (setup.sh) script. The Packer provisioner unpacks that TXZ file after uploading it and runs the script. The template below is for AWS but can easily be adapted to any other cloud provider, since there is very little that is assumed about AWS other than some conventions around subnets and VPCs.

{
  "variables": {
    "ami": null,
    "name": null,
    "description": null,
    "keypair": null,
    "subnet": null,
    "purpose": null,
    "vpc": null,
    "sg1": null,
    "ssh_keyfile": null,
    "instance_type": null
  },
  "builders": [{
    "type": "amazon-ebs",
    "region": "us-west-2",
    "ami_regions": ["us-west-1"],
    "source_ami": "{{user `ami`}}",
    "ami_name": "{{user `name`}} {{timestamp}}",
    "ami_description": "{{user `description`}}",
    "ssh_keypair_name": "{{user `keypair`}}",
    "ssh_private_ip": true,
    "subnet_id": "{{user `subnet`}}",
    "run_tags": {
    },
    "tags": {
      "Purpose": "{{user `purpose`}}"
    },
    "vpc_id": "{{user `vpc`}}",
    "security_group_ids": [
      "{{user `sg1`}}"
    ],
    "communicator": "ssh",
    "ssh_private_key_file": "{{user `ssh_keyfile`}}",
    "instance_type": "{{user `instance_type`}}",
    "ssh_username": "ubuntu"
  }],
  "provisioners": [
    {
      "type": "file",
      "source": "package.txz",
      "destination": "/home/ubuntu/package.txz"
    },
    {
      "type": "shell",
      "inline": [
        "cd /home/ubuntu && tar xf package.txz",
        "cd /home/ubuntu && ./setup.sh"
      ]
    }
  ]
}

Modifying it to work for other cloud providers is left as an exercise for the reader.