Hi Pete: Diesel has such low viscosity that it is very tough to keep contained. Tanks that are watertight will leak diesel fuel in some situations. The problem you will have in using the aft end of your keel sump for diesel will be getting a fuel-tight bond of the flange (horizontal and vertical), and then sealing to the horizontal flange with a lid. The diesel and fiberglass laminate should not have a problem. A lot of this depends on the space available in which to work. If you do try this here are a couple of things to keep in mind: 1-The fuel pick up should be several inches above the tank bottom so it misses any crud or water. 2-Have a second pick up to the bottom for cleaning the tank. 3-Any bolts used as fasteners to hold the lid down will require “O” rings under their heads for seal–otherwise the diesel will come up the threads regardless of what you try and do. There are bolts made which have a bed machined under the head for “O” rings. Be sure the “O” rings are diesel tolerant. 4-Test the tank with three PSI air pressure for 24 hours–and make sure it is air tight. Good luck–Steve