Try a spanner wrench. West Marine
sells one made by Groco. It's a very useful tool on any boat
. You will find many uses for it when other tools just don't work
. In the photo
on the West Marine
site it may not look like much, but it will easily open your diesel
fill cap and will handle virtually any fill cap configuration on any boat
. Go look at one in person and you will see exactly what I'm talking about.