to close the openings a releasable detent mechanism is provided in each of the support base transverse member posts 32 to releasably attach the posts 32 inside the openings of the body members the detent mechanism shown in fig8 and 9 includes a pin having a pin head 52 that engages through aligned holes 54 56 in the post 32 and the lower end of the body member 18 respectively to releasably secure the post in the opening of the body member the pin has a shank 58 formed integrally with the pin head 52 the shank 58 extends through holes in the conductive strip section 46 and the post interior wall 47 and thereby mounts the pin for reciprocating movement in the post 32 the mounting of the pin in the post hole 54 and the holes in the strip section 46 and interior wall 47 enables the pin shank 58 to reciprocate transversely through the strip section 46 and interior wall 47 and allows the pin head 52 to reciprocate within the holes 54 56 of the post and body member a spring 62 biases the pin head 52 into engagement in the body member hole 56 to securely attach the support base transverse member post 32 in the opening of the body member an annular collar 64 prevents the pin head 52 from passing completely through the post hole 54 by manually depressing the pin head 52 into the body member hole 56 the pin head may be disengaged from the hole 56 and the support base post 32 may then be disengaged from its insertion.