Slide out lockable safe for under rear seats which

[gpbst3]

I am looking for a lockable safe to go under the rear seat of my 19 crew cab. I would like to be able to secure a rifle under the seat. However there is a catch. I cannot flip the seat up due to having child car seats. I need the safe to slide open without lifting the seat. I did see one model but it was $1800 which is not happening.