Make custom record fields available in the inventory reports

1. Navigate to Customization > Lists, Records, & Fields > Item Fields > New.
2. Enter Label. 
3. Select on the Type field = List/Record.
4. On the List/Record field = select the custom record's list.
5. Check the Store Value checkbox.
6. Check the Record is Parent checkbox
7. On the Applies to tab, check Inventory Item
8. On the Display > Subtab, select Main
9. Click Save.

Note: Sign out and clear cache if it did not take effect right away.