model assertion assistance

I'm looking for some kind of README, wiki, example or other information on how to create the model assertion used as the argument for ubuntu-image. Also, is this model assertion all that is needed or does there also have to be a gadget.yaml file?

I'm trying to be able to script the creation of a full custom Ubuntu image that has the full UI. Debootstrap hasn't been working to get the full UI, so I'm hoping ubuntu-image can help. The repo's Manual Page doesn't give much about what is needed for the model assertion or other things needed to create an image.

Thanks.