Abstract: Development of Millimeter Wave Devices containing MMIC's is expedited by use of a new package for the MMIC's and associated electrical components that define the functional circuit. The package includes a base plate and a plurality of metal inserts removeably fastened to the base plate. The inserts are spaced apart and contain a profiled edge with the profiled edge in one in confronting spaced relationship with that of another of the inserts. Together with the backplate, the profiled edges define elongated cavities, serving as two of the cavities side walls. The cavities serve as a repository for the MMIC devices and some of the additional components and stripline. Should extraneous resonances be discovered, the insert can be removed and adjusted in size and profile, and replaced, thereby adjusting the cavity without disturbing the electronic components or MMIC's which are secured to the back plate.