Why three piece? Why not two piece and save a lot of time, work and weight?

But, there is no secret. You just need to build a drill jig to hold whatever you decide to drill with steady. The drill can be fashioned from a sharpened tube, a forstner bit or anything else that will cut foam instead of simply tearing it up. Do a search in the archives and you'll find several threads which cover this topic including diagrams of the drill jig.

John