The polystyrene was there to create a void in the concrete so that the bolts can go though from the base plate. Without the voids it won't be possible to bolt the plate tightly to the concrete!

We used a plywood template while pouring the concrete. This was an exact copy of the baseplate.

And yes you have to be damn sure that nothing is going to move when you pour the concrete in as the forces involved are quite massive.

ted