DrMilhouse, starting from the bottom up the bases are assembled like this, 1 walmart cutting board, 2 upside down drain grates screwed to cutting board from the other side, 2 thinwall 4" pvc drain pipe caps glued over the drain grates, 2- 4" sch 40 couplings glued to the 4" thinwall caps, the second set of grates just sit in the bottom of the couplings to keep the media off of the bottom, you will have to notch the internal grates for the pumps inlet and first to second chamber inlet. I hope this helps >>>>>>>>>>GEBC