There are at least two possibilities other than simple lock nuts. One is to buy a self-locking nut or a grub-screw locking nut. The other is to buy a pair of special lock nuts that are half as thick as standard nuts.
I'm not clear on the situation regarding the washers. You may need washers to enable the pins to slide smoothly in the slots. Lock washers on sliding parts are a no-no. Can you post pictures of this sliding mechanism?
