Angular2: best way to use FormBuilder Validators.required with a checkbox group

I'm building a form in Angular2 with FormBuilder & ngFormModel and cannot find an elegant way of validating that 1 or more checkboxes in a checkbox group are checked. I don't want to have to write a custom component method to validate checkbox groups everywhere I use them. Ideally it would be great to leverage the ngFormModel with FormBuilder & Validators.required or even a custom validator.