shown in fig4 a 4 b and 4 c each of the pivot pins 121 of the mount base 11 comprises an inclined free end 1211 which facilitates the engagement of the pivot pin 121 into the corresponding hole 122 on the respective support arm 10 preferably a chamfer 100 is provided on the second end of the support arm 10 to cooperate with the inclined free end 1211 of the pivot pin 121 so as to further facilitate the insertion of the pivot pin 121 into the corresponding hole 122 on the support arm 10 the support arm 10 also comprises retaining means 13 which retains the support arm 10 at the expanded position the retaining means 13 comprises a first projection 131 formed on each of the side walls 100 of the support arm 10 and engageable with a second projection 130 formed on the side wall 115 of the mount base 11 the projections 131 130 are shaped and positioned in such a way that when the support arm 10 is moved from the stowed position toward the expanded position the first projection 131 of the support arm 10 is brought into engagement with the second projection 130 of the mount base 11 and a further rotation of the support arm 10 causes a deformation on each of the projections 130 131 to allow the first projection 131 to slide over the second projection 130 thereby forming a tight engagement therebetween when the support arm 10 reaches the expanded position the engagement between the projections 130.