A standard powder bed fusion additive manufacture apparatus is provided with a device to modify the building space, including the powder feed for the building space, yielding a reduction in the active build area, with a concomitant reduction in the adjacent powder supply. A retrofit kit or assembly which is quickly emplaced on the existing equipment with little modification of the regular building space is provided, and is therefore readily removable when the building space is to be returned to its original condition. One advantage is the relatively quick manner in which the building space can be modified. A reduced area build piston is slideably received in a shaft depending from a cover that is emplaceable on the unmodified build area. The build piston moves with the pre-existing build platform. A like arrangement is provided for the feed container.