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Among the easiest techniques to build a case or bookshelf with adjustable racks is always to drill holes for shelf pins. While there can be a few manufactured rack pin drill jigs out, I more or less always make a custom template out of inexpensive pegboard. Small jigs that have to be moved several times to span the exact distance of the cabinet are notorious for giving you wobbly shelves because it's hard to make sure all four holes (two on each side) end up exactly level with one another