You don’t need any special tools for this. Simply go to a thrift store and pick a particularly thick book. Use a box cutter to hollow out the middle of the pages. Tuck your gun inside and slide it on a shelf next to a handful of other similarly sized books. Don’t go for an ancient, leatherbound medical textbook or some other stately looking book. Your best, most convincing disguise is an old Stephen King hardback that still has the dust jacket, or some other bestseller as thick as a brick.
This works equally well in the kitchen. There, people are expected to have some ancient, outdated cookbooks. Get an old, cheap edition of The Joy of Cooking or something equally dense, hollow it out, and sandwich it between a few newer cookbooks. This is better than trying to hide a gun in the flour or tucked in amongst the pots and pans because you won’t have to waste any time searching for it or fumbling through objects you actually use every day.