Your snake will probably do fine in a rack tub. Down sizing is less stressful than upgrading sizes.
28/31 qt tubs are great for snakes between 500-2000 grams.
41 qt tubs for large females over 2000 grams.
The only way a snake can escape a tub is only if you don't push it back into the slot. They can't push it out themselves.
You can secure it with locks and such like this:
http://s428.photobucket.com/albums/q...sort=ascending
Reptile Basics has these nubs on their rack that can come on and off that secure a tub in place:
http://www.faunaclassifieds.com/revi...eater_rack.jpg
Or you can make a swivle that turns and locks a tub in place like this:
(It's a rodent rack, but same concept)
http://www.rodentracks.com/images/Rat_Rack_4stack2.JPG