more effective seal between the sink bottom 20 and the vertical wall 14 as illustrated in fig2 the upper outer surface of the vertical wall 14 is provided with a plurality of lateral outwardly extending reinforcement or support members 31 which are secured to the wall 14 and underlie engage and reinforce the counter top 12 adjacent opening 16 the counter top 12 is secured to both the top edge of the vertical wall 14 and the reinforcement members 31 which extend at least partially throughout the length of each side of vertical wall 14 but do not extend completely around the periphery of the vertical wall 14 the support members 31 thus provide increased stability when the vertical wall and sink are undermounted in relation to the polymer based sheet material of the counter top 12 when assembling the sink of the present invention with a counter top in which the counter top 12 and vertical walls 14 are constructed of the same thermoplastic solid surface sheet material such as corian u00ae sheet material the wall components 30 of the vertical wall 14 are first cut with a circular saw or the like from the solid sheet material in the prescribed rectangular sizes the abutting side.