I had some large acess covers ( about 6" dia.) that always were a problem
with leaks and I had tried various rubber and cork gaskets and sealants.
Finally I did some research and deciced to use some aircraft fuel tank
sealant. I found plenty info on the internet but ended up going out th the
airport to and bought some from a local aircraft parts dealer. It was a two
part polysulfide compound. Since you have diesel (mine was gas) get one that
is approved for jet fuel. They have a flowable (class A) and a thixotropic
paste (class B) which I used. That stuff realy worked. I think this will
work for you as well.

--
Ron White
Boat building web address is
www.concentric.net/~knotreel