How to add a disk to VM when cloning from a template?

I've found pyvmomi examples on how to add a disk to an already existing VM, but I would like to customize the VM template and then clone. Setting the CPUs and memory are pretty straight forward, but the adding of one or more disks to an existing template, asides from the boot disk, eludes me.

# Add an additional 200 GB disk
new_disk_kb = int(20) * 1024 * 1024
disk_spec = vim.vm.device.VirtualDeviceSpec()
disk_spec.fileOperation = "create"
disk_spec.operation = vim.vm.device.VirtualDeviceSpec.Operation.add
disk_spec.device = vim.vm.device.VirtualDisk()
disk_spec.device.backing = vim.vm.device.VirtualDisk.RawDiskMappingVer1BackingInfo()
disk_spec.device.backing.diskMode = 'persistent'
disk_spec.device.unitNumber = 3
disk_spec.device.capacityInKB = new_disk_kb

# vm configuration
vmconf = vim.vm.ConfigSpec()
vmconf.numCPUs = 8            # change the template's cpus from 4 to 8
vmconf.memoryMB = 16 * 1024   # change the template's memory from 4 GB to 16 GB
# change the template's disks from
#    1 250 GB boot, 1 x 200 GB disk
# to 
#    1 250 gB boot, 2 x 200 GB disks
vmconf.deviceChange = [ disk_spec ]  # something is not right
                               
clonespec = vim.vm.CloneSpec()
clonespec.location = relospec
clonespec.powerOn = True
clonespec.config = vmconf
clonespec.customization = customspec
task = template.Clone(folder = destfolder, name = vmname, spec = clonespec)

The code works without the vmconf.deviceChange. Once I try to add a disk I see the error

Invalid configuration for device '0'.

or

Incompatible device backing specified for device '0'.

from How to add a disk to VM when cloning from a template?