Actually, with revolvers, it's really easy and really cheap:

Open the cylinder, and put a padlock around the topstrap. For obvious reasons, I recommend a key rather than a combination lock. The key can be put into the lock every night, but you have to remember to take it out in the morning, otherwise it is self defeating; either that, or hang the key in a near by but inconspicous place (like behind the headboard on a lanyard that you can find by feeling quickly). A high end padlock runs about 12-15 dollars, and if the cylinder won't close, a revolver is a poor club- safer than that is hard to do with a firearm.