noted that main radial slits 22 each has at least one secondary side slit 24 extending therefrom all secondary side slits 24 are substantially identical a further set of four radial slits generally designated by reference numeral 26 are located between radial slits 20 and 22 and separated from each by approximately 30 u00b0 radial slits 26 are substantially the same length as radial slits 20 and 22 it will be noted that radial slits 26 also include secondary side slits 28 however minor side slits 28 are located further from the distal tip end of radial slits 26 compared to the distance minor side slits 24 are located from the distal tip end of radial slits 22 the arrangement is such that a plurality of flaps 30 are formed by the arrangement of the main radial slits 20 22 and 26 and a plurality of tabs 40 are formed by the arrangement of the secondary side slits on flaps 30 as shown in fig2 slits 20 22 and 26 may terminate in small apertures 27 to thereby prevent tearing in the practice of the invention and as shown in fig3 5 and 6 a conventional round tube form generally designated by.