Rack Position in Custom Script

[robertlynch3]

I posed this on the Slack channel, but did not get a response.
I am looking to import a device via a custom script, I’ve got the ability to add the device to a Rack via

class DiscoverDevice(Script):
    site = ObjectVar(model=Site, required=False)
    rack = ObjectVar(model=Rack, required=False, query_params={'site_id':'$site'})

I’d like to add a Rack Unit (position) as well, but I can’t seem to figure out how to correctly do it so that all that is selectable is the available units in the rack