the towing ring 56 the different pairs of walls 54 58 have sufficient resilience to permit the insertion of the towing ring 56 by simply bearing in a direction substantially perpendicular to the bottom wall 30 the retention of the towing ring 56 is then ensured by the resilient walls 54 58 bearing under tension against the towing ring 56 moreover the bottom wall 30 may also comprise a substantially semi cylindrical recess 60 having the shape of the shank 560 of the towing ring 56 to remove the towing ring 56 from its housing 52 it thus suffices to exert traction in a direction perpendicular to the bottom wall 30 the walls 54 58 then deform to permit the passage of the ring 56 and then return to their initial shape the fourth housing 62 designed to receive a hub cap hook 64 is located between the conical blocks 42 the hub cap hook 64 takes the form of a rigid metal wire one end thereof 64 a the head 64 being curved so as to form a loop and the other end 64 b being rectilinear so as to form a shank the fourth housing 62 consists of a pair of clips 66 formed in the bottom wall 30 and aligned in a transverse direction and a second clip 68 located in front of the clips 66 the clips 66 are capable of receiving a terminal edge 64 c of the head 64 a of the hub cap hook 64 whilst the clip 68 is capable of receiving the shank 64 b of the hook 64 to store the hook 64 in its housing 62 therefore it suffices to introduce the terminal.