The precinct committee officers elected during the August primary will take office Dec. 1. They then have a month to finalize a slate of candidates. It's up to the party to determine if all the officers in the county or only those within the district now represented by Macleod will make those decisions.
Between the party central committee and the two county commissioners, there is actually two whole months to work after December 31. According to the state constitution, there is no need to appoint someone before Macleod even resigns officially.
Here's the language:
...shall be one of three persons who shall be nominated by the county central committee of that party, and in case a majority of the members of the county legislative authority do not agree upon the appointment within sixty days after the vacancy occurs...
This is backed up by the scuttlebut at the recent PCO orientation meeting this week. From what I heard, there will be an application process in early January. Those names will be considered by the PCOs in Macleod's district and then a (hopefully) long list of candidates will go to the entire body of PCOs.
Fittingly, this follows the path for county commissioners in wider elections. They are winnowed down during a primary in which only district voters can participate. But, the final decision is made by the entire county.