Tailgate locking mechanism?

[Southpa]

OK, folks anyone got any ideas for a locking mechanism for the tailgate on my 70 GMC? Over Xmas I found that my truck is easier to break into than I thought. I have a canopy (cap to you US folks) with a locking door on the truck. But I found that if you give a tailgate a strong pull it will pop out from under the canopy door. So I want to make the unit a little more secure short of breaking glass to get at the contents.

I have 2 options:

1. Add extra length to the bottom of the canopy door ie. pop rivet a strip of aluminum to add an extra couple inches.

2. Install some form of locking mechanism, or hasp or whatever on each inside corner. I guess they would have to be pop rivetted in as well but doesn't sound very strong.

I was just wondering of there was anything out there that was built for this function. Any other ideas are welcome. TIA