sealant for installing fuel tank deck filler

Does anyone have recommendations for what sealant I should use for installing a deck filler on top on top of a diesel fuel tank? It will be going into the cabin sole, and I'm pretty sure the tank is fiberglass (I won't know definitely until I put the hole saw into the cabin sole).

The screws will be going into fiberglass so they won't have a huge amount of grip, so I would expect the sealant would need adhesive properties and of course be resistant to diesel. The guy at Whitworths recommended some Stag Jointing Paste which I bought, but I suspect it won't have the adhesive properties needed. Any suggestions appreciated

I use Loctite 567 for everything from fuel to potable water lines.
www.henkel-adhesives.com/ee/en/product/thread-sealants/loctite_567.html

otherwise for fuel line only Permatex #3 works good.

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Ramona said..
If that boat in your signature is a Currawong then your tank is fibreglass. The new Selley's silastic will do or the Permatex product.
www.supercheapauto.com.au/p/permatex-permatex-fuel-resistant-gasket-sealant-85420/356517.html

Thanks for this Ramona, very helpful. It is indeed a Currawong. Do you know the capacity of their fuel tanks? The only access I have to the fuel tank at the moment is via a filler which is installed into the top of the cockpit seats which makes it difficult to inspect the fuel tank. So my plan is to install the filler directly onto the top of fuel tank so then I'll at least be able to have access to measure how much fuel is in it and do some fuel polishing etc