The Pods Framework allows you to easily create custom fields (among many other things). With this integration add-on, you can then create facets based on your custom field data.

Usage

After installing, browse to Settings > FacetWP and click the “Add New” button to add a new facet.

Within the facet’s “Data source” dropdown, you’ll see a brand new Pods section with all the fields you’ve created. Select one of those Pods fields to populate the facet with data from that field. Then make sure to Save and Re-index.