I will never place an HDD, be it 2.5" or 3.5" on the back of the motherboard in a vertical orientation, instead have the drive laying flat on a PSU basement or a drive caddy and make sure that there's airflow in the path of the drive. If you were working with a 2.5" SSD, that mounting location on the back of the motherboard tray is ideal.
I was in a similar situation with drive sleds for a case mod I'd wanted to do on a Coolermaster MasterBox Lite 3 case, whereby the caddy I was looking for didn't come in the case(by default from the factory), I reached out to the rep for Coolermaster and via email, I managed to get the sleds that were found in the MasterBox 5 case. You might want to contact Antec and see if they can hook you up with the sled you're asking for, though please keep in mind, having an HDD on the back is a bad idea.