pair of side blocks 202 204 and a thin walled attachment plate 206 that extends between the side blocks 202 204 centrally in the base 130 the support block 126 is fixed to a rear surface of the side block 202 whereas the support block 128 is fixed to a rear surface of the side block 204 the attachment plate 206 has a square recess defined substantially centrally in a rear surface thereof the square recess which is defined in the attachment plate 206 makes the attachment plate 206 into a main attachment plate body support 212 and a thin walled panel pressure boosting unit deformable portion 214 which is disposed on and formed thinner than the main attachment plate body 212 in other words the main attachment plate body 212 is positioned around the thin walled panel 214 as shown in fig1 the rear surface of the main attachment plate body 212 has a dent 216 which is defined substantially centrally widthwise in one end portion of the main attachment plate body 212 and is joined to the channel behind the thin walled panel 214 the thickness of the portion of the main attachment plate body 212 in which the dent 216 is defined is greater than the thickness of the thin walled panel 214 the thin walled panel 214 is positioned behind the cavity forming portion 176 see fig1 the thin walled panel 214 has a plurality of through holes 138 defined therein the thin walled panel 214 is lower.