Review on NDS 12 in. Square Catch Basin Drain & 12 in. Low-Profile Adapter Drain Riser by Jasmin Rojas

If you installed the bottom box parallel to the slope, this is ideal for an 8" extension, but you may not need an 8" deep or One side needs to be lower to accommodate the slope of the tube I'd happily pay more for an adjustable extension I can't be the only one thinking about this I can't find in-between sizes I cut it at an angle to get the right value Get silicone in place and some cement on the outside of the box at the seam below the level. If you just fold it up there is a hole to screw the extension to the box but once you cut the extension off , there are holes for mounting screws. Hope this helps.