Hypothetical question ref leaking diesel tank (may be a real question):(

[Bloater]

If you use expanding foam to fill the void then you will probably never be able to sell the boat afterwards. Someday you will want to sell and every penny you think you are saving now will be lost in the future when the purchaser knocks you down.

I think fixing it properly is the only way to go, sorry.

 

[Otter]

Might this work?
http://www.thomson-medium.co.uk/

That medium may well be thick enough.

 

[CAPTAIN FANTASTIC]

As some have suggested already, can you cut out part of the top of the tank to get access to the inside of the tank; this will give you the opportunity to line-up the tank with a membrane or inset a bespoke flexible tank.

In theory, if you fill the gap with epoxy or similar, it should work, however, in practice, because of the sea motion, diesel oil will find its way out and over the tank; then you will have an ever bigger problem. Also, tanks corrode on the bottom corners and on the underside, which is difficult to repair without taking the tank out or fixing it from inside.