The yeast used can affect on the taste. The Finnish baking yeast is quite effective and it will give quite a sour taste. I don't know how beer yeasts will do. I believe that those would do well. Anyway the sourness is quite characteristic for sahti.
All instructions given above are approximate.
Sauna and sahti I was also asked about suggestions how to use sauna in brewing. Besides, sauna has been traditionally considered as the cleanest place of a Finnish household.