concave surfaces of protrusions 24 and 60 provided within receptacles 10 and 10 u2032 to that effect the foot portion 16 includes a substantially convex portion 148 having a radius of curvature less than or equal to a radius of curvature of the concave surface 144 as shown in fig6 b the foot portion 16 includes in a particular embodiment of the invention a block of material 150 including the substantially convex portion 148 block 150 is attached or connected to the frame 12 fig2 a and 23b illustrate alternative frames 812 and 812 u2032 including alternative foot portions 814 and 814 u2032 foot portions 814 and 814 u2032 have a cross section similar to the cross section of the foot portion 16 and each present a respective convex portion 818 and 818 u2032 the foot portion 814 extends substantially across the whole frame 812 while the foot portion 814 u2032 extends only across part of the frame 812 u2032 in specific embodiments of the invention foot portions 814 and 814 u2032 take many different forms for example foot portions 814 and 814 u2032 can take forms similar to the forms described below with respect to alternative forms of the foot portion 16 fig6 c and 6d show two alternative blocks 152 and 154 both blocks include a substantially convex surface 148 block 152 is similar to block 150 except that block 152 includes a substantially prism shaped extension.