Since Kaijus don't really use the extra deck all too much there wasn't really a need for one outside of shenanigans with Banquet of Millions, which is why it's side-decked, to lock your opponent out of using theirs for the turn.
If you do use Banquet of Millions, just use any random 15 extra deck cards you happen to have, like Ojamas or anime waifus.
I'm a huge Godzilla fan, so I just wanted to make a cheap kaiju deck for fun.