So just finish drawing the edges of your ramp. Draw parallel from the trench drain until you hit the corners of the walls. Then extend the slope down from where you stopped to the corner. Then select all the lines and sandbox from contours.
Then go into the sandbox group and turn on your hidden geometry and delete the extra bit that filled in around the corner.
Here it is with that little bit removed.
Then, if you want ramp 6" thick, use FredoTools Joint Push Pull, specifically Vector Push Pull to select the ramp face and extrude it up or down 6".
