for the two lines running along the top of the trench, I'm guessing they're covered in black stuff. Put your battery in while the trench is empty and let it heat up. Very carefully, take a knife or scissors or something pointy and very very carefully scrape it off, leaving the battery in the whole time. It will come off easily when the trench is super hot, but if its room temp its stuck like glue.
As for the rest, carefully stick a pokey tool into the "mouth hole" and try to scrape it out. Harvest it, it'll get you high.
Be VERY VERY gently when doing this type of cleaning.