Reddit reviews TACKLIFE Pop Rivet Gun, River Gun with 40Pcs Metal Rivets, 4 Replaceable Nozzles for Metal, Wood and Plastic - HHR1A

Remember that fridges all have hardware attachment points for door handles and hinges, and that those screw points can be used. The non side-by-side fridges have provision for reversing the hinges, so there are very sturdy hinge mounting points that are just sitting there unused on the handle edge.

It won't keep out a determined burglar, but some d-ring-hangers as for artwork can often have additional holes drilled that will line up with fridge handle screws -- and can be mounted while leaving the fridge door handles in place. That gives a spot to look a cable or small chain and travel-lock.

The most elegant solution I've seen was at a house where the parent removed and stored the grill from the bottom front of the single-door fridge, and then made a wooden spacer in order to make the bolt for a patio door aux lock line up with the vestigial 3/8 hinge-pin-hole on the bottom edge of the door. The screws to mount the wood block were all in a spot that would be behind the grill when it is replaced (and you can see what's behind the flange, so you know it is safe to drill!), and the lock mounts easily to the wood.

EDIT: A similar thing can be done with mending plates that have a plunger lock pop-riveted to them –– mount the mending plates to the unused hinge-bracket holes at the top and bottom of the fridge, lining up the plunger to fit the hinge-pin hole at the top of the freezer door and bottom of the fridge door. Will require drilling the mending plate with holes that match hole positions on the fridge and lock.

If the existing handles are loop handles and it is a side-by-side, there are adjustable cable locks. If they aren't loop handles, it may be that replacement loop handles area available for a sister-model of the fridge and will bolt into the same holes.

If the vestigial center hinge-bracket holes have a hole that is visible through the slot between the fridge and freezer doors, you can use that hole for a matching-thread-size eye-bolt long enough to put half the eye exposed in front of the door - then a padlock through the eye will stop both fridge and freezer from opening, and the portion of the eye trapped between the two doors will stop the eye from being turned to remove it unless both doors are open.