and blocking gas distribution in the present invention a heat sink concept is utilized by the provision of an elongated metal strip preferably stainless steel contained within each of the slots 22 fig8 shows a strip 50 constituting a strap like configuration with a flat elongated body portion 52 having opposite planar surfaces and an integral end tab portion 54 projecting at a right angle to the body portion 52 a strip 50 is placed to extend substantially the full length within each slot 22 as shown in fig9 whereby the end tab 54 rests against the base end 18 of the plug 12 fig1 illustrates the base end 18 of the plug 12 with a strip 50 disposed in each of the slots 22 whereby the respective end tabs 54 overlie the surface of base end 18 of plug 12 in a direction lateral to the opening of the slot 22 each strip 50 at least one third and preferably about one half the available area within the slot 22 with the width of the strip being such that it extends substantially the full depth 34 d 34 of the slot 22 as shown in fig4 and the thickness of the strip being one half the slot width 34 w 34 the presence of the strip 50 with the slot 22 and its full expanse.