A Couple of Different Methods
Easiest is to drill a hole thru the latch tab that sticks outside of the fuselage when the latch is engaged. A small padlock thru the hole prevents the latch from being opened. I used a metal padlock and added heat shrink around the body to prevent scratching the paint.
I've seen some keyed locks that engage the latch from the inside - much more work to install, adds weight, but looks good.
Remember, if someone wants to get into your plane, they will. Some builders leave the locks off entirely with that thought in mind - better to lose some "stuff" than risk a shattered canopy.
Terry, CFI
RV-9A N323TP