How can I create a "Custom Work Field"

I would like to implement custom fields which display a work value, for example "20 d". The fields can be either calculated (e.g. the work of a specific resource group), or entered (e.g. an initial estimate of work, which should not be entered in the standard "Work" field yet).

How can I create a field which displays the values including the units (e.g. "d" or "hrs"). I could take a Custom Number Field (but then I cannot add a unit, as far as I know), or a Custom Text Field (but that's not really what text fields are for), or tweak a Duration field for that purpose). What would you recommend?