"Grouted" the counter last night. Used black RTV silicone so it have a bit of movement.
Tape off all the edges.
Work it into the crack, then wipe as clean as possible down the tape line. Carefully pull the tape.
Then I crawled under to finish mounting the poop tank. Added large fender washer to the screws.
Now a question. This section of the tank is not supported. I hung off the back corner and it didnt really seem to flex much at all. Is it worth making a bracket to hold that back corner? Its 1/4" abs on that top flange.