18" tall box. sometimes i wish it was bigger but if i lift up the end a bit with like bricks it makes up. i would make it 24" two feet even would be good. make sure its stable though, dont go to heigh and to narrow where it will fall a ton.
my kicker rail is 1.5', my flat its like 10" and i wish it was taller (and for sturdy at that.) and my battle goes from like a foot and 2 inches or something to 3 ft