Yes that's the idea, foam it in the way it was. I cut about an inch from the port side and aft to get it out so would want to fill to restrict movement of the tank.
I suppose with it strapped down there wouldn't be much movement, or I could drop plastic inserts in to take up the space. That would improve ventilation all around.
Noted on the 3M UV 4000, I'll pickup a couple tubes. Step one is to get some gas in there to make sure everything is working properly before sealing it up!