This code can slip right into user-data for an AWS ec2 instance for both win2k8 and win2k12r2 as is to stripe your ephemeral volumes. This works on more than 2 ephemerals as well (tested on a d2.xlarge with 3 1.8TB drives)

windows_2k8_2k12r2_ephemeral_stripe.ps1

<powershell>
#################################################
#  Detect the Ephemeral drives and stripe them  
#################################################

$DriveLetterToAssign = "Z:"                           # Be sure to choose a drive letter that will not already be assigned

#################################################
#  Given a device (e.g. xvda), strip off "xvd" and convert the remainder to the appripriate SCSI ID   
#################################################
function GetSCSI {
	Param([string]$device)
	
	$deviceSuffix = $device.substring(3)      # remove xvd prefix

        if ($deviceSuffix.length -eq 1) {
		$scsi = (([int][char] $deviceSuffix[0]) - 97)
	}
	else {
		$scsi = (([int][char] $deviceSuffix[0]) - 96) *  26 +  (([int][char] $deviceSuffix[1]) - 97)
	}

	return $scsi
}

#################################################
#  Main   
#################################################

# From metadata read the device list and grab only the ephermals

$alldrives = (Invoke-WebRequest -Uri http://169.254.169.254/latest/meta-data/block-device-mapping/).Content
$ephemerals = $alldrives.Split(10) | where-object {$_ -like 'ephemeral*'} 

# Build a list of scsi ID's for the ephemerals

$scsiarray = @()
foreach ($ephemeral in $ephemerals) {
	$device = (Invoke-WebRequest -Uri http://169.254.169.254/latest/meta-data/block-device-mapping/$ephemeral).Content
	$scsi = GetSCSI $device
	$scsiarray = $scsiarray + $scsi
}

# Convert the scsi ID's to OS drive numbers and set them up with diskpart

$diskarray = @()
foreach ($scsiid in $scsiarray) {
	$disknumber = (Get-WmiObject -Class Win32_DiskDrive | where-object {$_.SCSITargetId -eq $scsiid}).Index
	if ($disknumber -ne $null)
	{
		$diskarray += $disknumber
		$dpcommand = "select disk $disknumber
	                        select partition 1
	                        delete partition
	                        convert dynamic
	                        exit"
	    $dpcommand | diskpart
	}
}

# Build the stripe from the diskarray

$diskseries = $diskarray -join ','

if ($diskarray.count -gt 0) 
{
	if ($diskarray.count -eq 1) {
		$type = "simple"
	}
	else {
		$type = "stripe"
	}
		
	$dpcommand = "create volume $type disk=$diskseries
	                 assign letter=$DriveLetterToAssign
		         format fs=ntfs quick
	                 exit"
	$dpcommand | diskpart
}
</powershell>