below the mounting unit 14 may be simply comprised as a plate 21 having an adhesive 22 on the back side thereof and a pair of spaced lugs or projections 23 and 24 extending from the front face thereof the adhesive 22 may initially be covered by a waxed paper or the like 26 which may be peeled from the adhesive for use of the latter in attaching the mounting unit 14 to a bumper 12 the holding unit of the present invention as illustrated in fig3 and 4 for example is formed as a rectangular frame or case 31 having a back wall 32 from which project a pair of lugs 33 and 34 spaced apart to fit over the mounting unit lugs 23 and 24 the lugs 33 and 34 of the holding unit 16 are provided with aligned lateral apertures therethrough adjacent the outer ends thereof and similarly the lugs 23 and 24 of the mounting unit 14 are provided with aligned lateral apertures therethrough adjacent the outer ends thereof in the illustrated embodiment of the present invention the apertures in the lugs 23 and 24 are aligned with the apertures in the lugs 33 and 34 with bolts 41 and 42 extending through the lugs 33 and 34 into threaded engagement with the lugs 23 and 24 so as to secure the mounting unit 14 and holding unit 16 together in pivotally adjustable relationship it will be seen that the holding unit may be pivoted around the center line of the wing nuts 41 and 42 as indicated in phantom in fig3 this is particularly advantageous for installations wherein the surface of a bumper may be inclined from vertical considering.