CGEL* has this on page 149:
The open conditional is the default type, while remote conditionals have the following properties:
 i Subordinate clause: must contain a preterite (or irrealis were) expressing modal remoteness.
ii Matrix clause: must contain a modal auxiliary, in the preterite if possible.[Footnote]
Footnote: ‘If possible’ in [ii] allows for the occurrence in the present tense of those modals like must which have no preterite form (cf. §6.2.1).
In CGEL's terminology, an "open conditional" refers to a conditional construction (If P, then Q) where P and Q are presented as an open possibility; a "remote conditional" refers to a conditional construction where P and Q are presented as a remote possibility.
What CGEL says above is, in "If P, then Q" in a "remote conditional" type, Q can contain the modal must. But I can't seem to find a remote conditional example where Q has the modal must.
Unfortunately, CGEL's §6.2.1 fails to show any example of a remote conditional whose Q contains the modal must.
Can anyone think of a remote conditional example whose Q contains the modal must?
*The Cambridge Grammar of the English Language