of each peripheral wall opening 28 preferably extend longitudinally and parallel to each other with the upper end of each peripheral wall opening 28 being preferably rounded or actuated the peripheral wall openings 28 divide the compression fitting 22 into four quadrants whereby each peripheral wall opening 28 is separated from another peripheral wall opening 28 by two stiffening ribs 24 the stiffening ribs 24 and the support ribs 26 provide strength and rigidity to the support member 20 extend longitudinally along the inner sidewall of each support member 20 and are operatively connected or preferably integrally molded to the support member 20 preferably eight stiffening ribs 24 and four support ribs 26 extend along the inner sidewall of the support member 20 as illustrated in fig8 and 9 the support ribs 26 are disposed at ninety degree angular intervals the stiffening ribs 24 are preferably disposed between support ribs 26 such that there is a thirty degree angular interval between each stiffening rib 24 and between a stiffening rib 24 and a support rib 26 both the stiffening rib 24 and the support rib 26 are preferably frustoconical in shape fig3 illustrates a plurality of mats in stacked nested relation for reference these mats are labeled mat a mat b mat c and mat d each mat is preferably substantially identical to each other mat and is constructed according to the principles outlined above as has been illustrated the substantially open receiving end 21 is adapted to receive the compression fitting 22 of the support member directly above for example the compression fittings of mat b are axially inserted into the substantially open receiving end support member of.