A fixture for use in installing a unit within a recess having spaced structural members, said fixture comprising a holding member for carrying the unit, mounting means operatively secured to said holding member and including means for varying its length so as to permit said mounting means to be extended between the structural members, and means operatively and movably secured to said mounting means for fastening said mounting means to the structural members, said fastening means being movable between a first position, in which the length of said mounting means can be varied, to a second position in which the length of said mounting means cannot be varied.