This kind of 5 day fast is also prescribed for the last 5 days of Kartika (Damodara) Month and it is known as "Bhisma Pancaka". During that vrata puja offerings to the Lord are also prescribed. I am listing them below. There are also mantras that go with these offerings. However, I didn't find any specific mention about this for Purusottama month. In the Padma Purana it only stated that in addition to ekadasee one can observe a "pancaratrika" fast until amavasya. Here are offerings prescribed for Bhisma Pancaka.
Devotees may offer the following to the Deity form of the Lord:
1. On the first day, one must offer padma (lotus) flowers to the feet of the Lord
2. On the second day, one must offer bilva (wood-apple) leaves of the thigh of the Lord
3. On the third day, one must offer gandha (scents) to the navel of the Lord
4. On the fourth day, one must offer java flower to the shoulders of the Lord
5. On the fifth day, one must offer malati flower to the head (siro-desa) of the Lord