You gotta buy the locking header and its mate for the PCB.
Take a quick look at the Molex website and find what you've got and what you need.
Molex is popular because they're relatively cheap.
Take a few minutes to look at what else is available from them, then decide what you
want to do.
P.S. some rubber bands chemically interact with insulation on wires, and can screw you up.
Also, if the vicinity of the board gets hot, your rubber band won't last long.
If you just have pins on your board itself, it's not all that difficult to replace it with a
locking connector.
Good luck. I'd use the 'rubber' for other purposes.