The sauna will sit on concrete piers formed from cardboard 'sono' tubes. I'm using 8" tubes for this project. 40 years ago I had a post hole digger, strong back and lots of energy...an not so much money. Today, the rental of a BobCat with a 12" post hole attachment made more sense.

You can't see it in the pictures but I laid out the locations for the piers based on my design drawings. Ultimately I wanted the concrete piers to be hidden by the sauna's skirt, leaving a drip edge along the bottom course of logs. All of this has to be thought out very carefully when you design your sauna. It's kinda hard to change the location of a 4ft. 8" dia concrete pier later on in the process. I did buy some pier footing forms, those black funnel looking things. The ground however turned out to be very hard packed and after looking at some load bearing tables it turns out I didn't require the extra surface area provided by the forms to hold the weight of the finished building. On this job there was no need to use them.