Custom Header Format object in OOT module

I'm trying to define my own customer header format object by inheriting from header_format_base but when i try to use it I get the following errors depending:

If just instantiating without namespace

value "header_format_custom" can not be evaluated: name "header_format_custom" is not defined.

if instantiating with OOT modules namespace 

'oot.header_format_custom' cannot be evaluated:

module object had no attribute 'header_format_custom'

Tried creating a variable block for grc similar to the Default Header Fmt Obj. block

I'd rather keep this in an OOT namespace than modify a local gr-digital.