Good suggestions, although I'd make two others: 1. You may not want to glue the lid onto the sump pump. Just placing it there should do the same job, and you'll be kicking yourself the first time you want to get in there to check something, if you glue it in place. 2. Check with your local swimming pool suppliers. They should have a rubberized gap filler for pool decks, available in larger containers. A standard caulk tube is 10 oz. The larger one you're discussing is 29 oz. = 52 cu.in. Now figure your gap is likely 2" wide, and you'll be filling "an inch below the floor". So, 2 cubic inches per linear inch... or one large tube of caulk per 2 feet of perimeter! You can buy this caulk in 1 or 5 gallon buckets from a pool supply house.