[erebus]

You can use neoprene, or pretty much whatever you have on hand if its a poly tank.
Neoprene is standard with aluminum tanks, because the neoprene will not react with the aluminum if any salt water sneaks into the tank area.
Because you're using poly you can use pretty much anything.
I installed my poly tank in my 18 with some cleats I made out of starboard.

The only thing to keep in mind is that a poly tank will increase in size slightly when you fill it with gasoline.
This is a permanent increase so you need to account for it when you install it.
Its approximately 3% in all directions.
i.e. dont install the tank so tight in the coffin that when it expands it will crack the fiberglass.