battery enclosures - i am not geting the hang of it
You will generally have to look at the manufacturer's documentation to get the real details of enclosures (or almost anything else).

Your first link, to the Gainta box, has a link to a technical information document which states that there are options for a battery compartment that can holds 2 or 4 AA cells. According to that same document they have the available metal clips for the battery compartment contacts which have to be ordered separately. From the looks of it these contacts are designed to mount to one end of circuit board.