Adhesive for mat to top of tank suggestions

3m adhesive (the spray is a good idea @CFD244 ) You can use a few dabs of 3m weather-strip adhesive. I would use the black but the yellow would work fine. You can lube the fill tube and grommet with anything slippery. Something that washes off with water is my choice. Dishwashing liquid works well.
Excellent ideas here gentlemen! Thank you for the feedback! Very helpful and glad I asked!