Best screws to attach WH to foam board?

If you have room, PUSH the water heater further in the compartment. If no room pull the Water Heater. Then glue 2X2 wood studs up the sides and across the top on the inside to that foam board. Then once the glue dries, pull the water heater back to the correct install in the hole. Install screws to hit the new 2x2 backers and make sure you install 2 screws thru the BOTOOM if not already installed. Then seal the corner gaps with normal expanding spray foam and install the outer trim ring with its screws. The water heater normally just has the 2 screws thru the bottom into the floor, but it is best to install screws on the sides also. There is no real force that will cause the WH to attempt to slide outward as long as the 2 floor screws are installed. USUALLY, the OEM's have a aluminum or small wood studs that frame the hole cut out for the Water Heater inside the foam wall before it is laminated. I have even seen where they install aluminum angle from the inside, but the wall is usually thicker to do that. Doug