a ball and socket type thing.. cant think of the exact name but its a almost flush receiver plate and stud with a plastic ball. . you have to push down for it lock and a good tug to pull back up
i cant find the unit i used but the pics show roughly what im talking about
what about putting a plastic c clip Maclow clip or similar as in picture 3.. on the seat top and making an oblong hole on the base.. make the oblong smaller and when the c lip touches it it squeezes in and locks it down.. this is what i use to lock down sea-rider seats and works very well. there are various sizes available.
very simple idea and works well