While it's now too late to pre-order the Fallout 4 Pip-Boy Edition, and thus get your hands on a neat, official replica Pip-Boy I dare you to wear out in public, thanks to an enterprising 3D printing designer and Fallout fan we can now have the next best thing. Yvo de Haas has released, for free, a schematic for the Pip-Boy 3000 Mark IV. You can see it above, and in addition to accepting your iPhone like the Bethesda one can, it will also hold a cassette tape. (It won't play it though.) While it's not quite as fancy as the Bethesda-sanctioned wrist-weight, it is pretty close.
All you'll need, once you've downloaded the schematic, is access to a 3D printer, and who doesn't have access to one these days? Oh right, everyone. You'll also need screws, glue, sandpaper and other parts, along with the tools to put everything together. All told, it will require a bit of effort, but if you already own a 3D printer you probably have a lot of the necessary stuff lying around.
Haas is currently working on an "authentic" version of this new Pip-Boy, which should be more accurate to the model seen in Fallout 4, but that won't accept your iPhone. It's not Haas' first time making a Pip-Boy either: here's a link to a version based on the Fallout 3 one. (Cheers, Eurogamer.)