We present how it is achieved to mount a double prism in the filter wheel of MIRIM - the imager of JWST's Mid Infrared Instrument. In order to cope with the extreme conditions of the prisms' surroundings, the low resolution double prism assembly (LRSDPA) design makes high demands on manufacturing accuracy. The design and the manufacturing of the mechanical parts are presented here, while 'Manufacturing and verification of ZnS and Ge prisms for the JWST MIRI imager' are described in a second paper . We also give insights on the astronomical possibilities of a sensitive MIR spectrometer. Low resolution prism spectroscopy in the wavelength range from 5-10 microns will allow to spectroscopically determine redshifts of objects close to/at the re-ionization phase of the universe.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.