Removable Table Top for Workbench
Generally, the first step in this type of project is to decide on the hardware, then design the unit around the hardware.

A general idea would be to use cleats mounted underneath the top to keep it square on the bench, then use hardware to latch it down ....

1) A couple of gate hook and eyes mounted underneath:

https://www.google.com/images?q=safety+gate+hook 

2) A couple of swivel hasps mounted underneath:

https://www.google.com/search?q=swivel+hasp 

3) A surface mounted deadbolt w/key, mounted underneath:

https://www.google.com/images?q=surface+mounted+deadlock 

4) A couple of window sash locks or clasps, mounted underneath:

https://www.google.com/images?q=window+sash+lock

https://www.google.com/images?q=child+proof+sash+lock 

For the workbench top, I recommend a used solid core door from, say, HH's Restore. Cut it to size. It's weight will help make the bench stable.