We used a mini excavator, and dug a "way too big" trench.....

We had hundreds of blocks of this insulation, so we laid a base down, and then built a form, and backfilled with chips...

Not the simplest solution, but we used what we had available. I thought about the trencher also, but each post would have been an obstacle.

I really wanted insulation on all sides of the concrete, and this method did accomplish that. 

You can see the whole project here...