Slip a padlock into the brass clasps you find on most hard-shell guitar cases. Many times, the central clasp on the case features a small loop that you can seal shut with a simple lock that you can find at any hardware store. Soft-shell guitar cases are less likely to have lock loops, but they often are closed with a zipper. The thin ring of a luggage lock will fit through multiple zipper tabs.
Purchase a hasp from a hardware store. This is a hinged piece of metal hardware that closes onto a loop. Be sure that your hasp comes with the screws necessary to affix it to your guitar case. The hardware store will likely have a wide variety of hasps available; consider bringing your guitar case to the store so you can select the one that looks as though it will fit best. One good thing about installing your own hasp is that they can generally be purchased at reasonable prices. You also have a number of decorative options, such as the variety you find at Bic Warehouse.
Remove the hasp from its packaging and hold it up to your guitar case in the position in which you wish to install it. Use a pencil to mark where you will make the starter holes for your screws. As you're installing small screws, you may want to follow the advice given by Commando Products and use a very small bit.
Drill the screws through the hasp and into the case. Flip the hinge of the hasp up and down several times to make sure it is secure and flips easily. (If it sticks, you might squirt a little multipurpose oil into the hinge.)
Slide the ring of a padlock into the closed hasp and slam it home. Whether you use a dial-controlled combination lock, or one opened with a key, you have given your guitar just a little more protection on the road.