Often times I hear from people that they have a hard time placing their phone on a bike in such a way that they can still use it as a GPS, so I decided to make a custom mount for my friend's phone that could be attached to his bike via either zip ties or Velcro straps connected to hooks on the back of the case.
Step 1: Find the Basic Dimensions of Your Phone
The first step for making any part that has to interact with something that already exists is figuring out how big the other thing is. In this instance we are making a modified phone case and so need to know basic information such as the length, width and depth of the phone. I used a dial caliper to find the dimensions of the phone I used but less precise tools could work (just remember to error on the side of a case that is too big rather than one that is too small).
Another useful thing I have found is to build in about a 1/16 of an inch on every dimension because trying to fit a phone into a case that is exactly the right size is hard and often times a printer will print ever so slightly smaller than the specified dimensions due to inaccuracies in the printing process.
The handiest way I've found to remember the dimensions of an object is to sketch a rough 2d version of it and label the different parts in a similar fashion to real engineering drawings.
Step 2: Find More Specific Details
The size of your phone isn't the only consideration for a case. Other important pieces of information include how far the screen is from the edge of the phone (so you can prevent blocking portions of the screen), where the power button is, where the camera is, where the charging port is (if you wish to use a portable battery) and where the headphone jack is (if you wish to use headphones).
The other important dimension to find is the outer portion of the case. I have found that .15 inches works well for cases as it doesn't become excessively bulky and still hold together with repeated use. Whatever case thickness you choose to use add that to each side of your phone's dimensions to get the case dimensions (remember it is .15 thickness on each side of the phone so it actually increases the dimensions by .3).
Step 3: Open Your CAD Program
To create a 3D part you need to model it in something called a CAD program. I used Solidworks as it was provided to me by my school and I have had the most experience with it but Tinkercad is another great program that doesn't require a download and is completely free.
Remember, if you are not familiar with the program it might be best to follow the built-in tutorials rather than jumping headfirst into your new project.
Step 4: Design the Case (the Next Few Steps Are Broken Down)
Here I have a video of creating the case; the next few steps break down what I did here.
Step 5: Create the Outside Shell
Create a block the same size as the outside of the case you determined earlier. In Solidworks this is done through drawing and extruding a rectangle whereas in Tinkercad this is done by dragging in a block and reshaping it.
Step 6: Cut Out the Top Slot and Screen Access
Cut out a rectangle at the top of the case to insert your phone remembering to leave the thickness at the bottom of the case. Cut out another rectangle the size of your screen (we figured this out when we found out how far the screen was from each edge).
Step 7: Add Cuts for the Power Button, Camera and Other Important Bits
Depending on what parts of the phone you want access to cut in different slots in the case.
Step 8: Add Brackets
The brackets on the back have the same thickness as the rest of the shell and are the crucial bits to allowing the phone to be mounted on a bike. The curves are obviously unnecessary but more aesthetically pleasing.
Step 9: Save
Save the file as both a Soldworks part or Tinkercad document as well as an Stl file (the file printers use) so that you can make modifications if something isn't right.
Step 10: Print/Assemble
Gaining access to a 3D printer isn't always easy but many libraries now have them and personal printers are coming in the near future. You can either print the case as one piece with supports or as multiple pieces (requires carving the part into multiple parts) and gluing them together. I've tried both ways and found they can be about equal work.
Step 11: Modify and Repurpose
The bike mount I made for my friend came from a design I originally made for a tripod mount for my phone and the basic case structure can be re-used to make a variety of different attachments for phones (and if one were so inclined a "multi-tool" of sorts).
The greatest feature of this is it's simplicity and it's malleability in the sense that the design can be re-made for whatever you need it to do, not just it's original purpose.