You could try making a copy of the PR Check form and renaming it to the PR_DirectDepositStub and go from there.  Not sure how that will turn out as there is no DD printing option for Check, stub, stub form.  Good luck.  As you can tell from the lack of responses, it might be easier to accept the change.

I recently had the same issue.  The default form in designer is Stub, Stub, Check, Stub, Stub.  Do away with the first two stubs and this  will lock your form type to  Check, Stub, Stub.  The trickeist part is adding the new fields to the form.  You add PR_CheckPrintingWrk then replace all the old fields with the new fields.  Once you have this complete remove the old tables and name it as btmlinesoft pointed out, PR_DirectDepositStub.rpt.  I have seen issues with locking the form down as I suggested, you may get extra headers (check portion) after a correct DDStiub.  This is fixed by using suppression on the section like  "IF NEXT({DDSpell Amt}) = {DDSpell Amt} then TRUE"

My comment is Sage sure made it really hard for clients and partners to move up after 2017.  Forms should not be this complicated.