how big?
the best kind of vivarium is the wooden type with two sliding doors, as these can have a lock fitted
http://www.ssnakess.com/forums/showt...threadid=21125
theres one on here.
they are good because they do not stress the snake from being over exsposed, and you dont really need a lock as long as you shut the doors properly, but will be safer.
if the snake is to small it will need a smaller tank then it would if it was fully grown (as it will feel to exsposed), but the general rule is 1 square foot for every foot of snake and 1 third of the snakes length high (or bigger) so a 4' snake needs a 4' squared floor space and 1 and a bit feet high viv. my adult cal king is in a 3' X 1 1/2' viv and hes happy