Welcome! I must warn you I am a little rusty when it comes to making Instructables, so please bear with me.

Let's start with a little context: I have been biking A LOT recently and switched all my luggage (cliff bars, water, wallet, phone, etc) from my backpack to my bike. It has been absolutely wonderful. Not only was my backpack literally wearing down a hole in my jersey, it also trapped heat, leaving me with a sweaty back. Unfortunately, there was not enough room on my bike to store everything and so I have been leaving my first aid kit at home. This isn't such a big deal for people who go on shorter rides and/or ride through densely populated areas. However, I ride remote gravel and/or 100+ miles every weekend. It's just not safe!

The solution: Get more on my bike, aka, another frame bag. I would happily buy another, but I already have bags in all the conventional locations! After examining my bike and a little inspiration from the Specialized SWAT storage box I decided the best place would be the bottom of the front triangle. If you aren't familiar with bike terminology, this is basically between your feet next to the gear.

My first attempt at producing the box (vacuum forming) didn't end up working but I have included it anyways because it's rather interesting. Feel free to skip it if you're pressed for time. However, I imagine you've got some time what with this pandemic and all.

Supplies

3d Printer
Zip tie

Step 1: The Geometry

There are a few ways to get accurate frame geometry:

CAD from the manufacturer
- I have never heard of a bike company giving out CAD models of their bikes, but many hardware suppliers do, so it's theoretically possible although highly unlikely.
CAD from someone online
- I actually forgot about this one until I started writing this :/ This too would probably be hard to find though.
By hand
- You could theoretically use a contour gage and an accurate jig to get contour data from your bike every X inches and then use the loft tool to replicate your bike. This would probably work really well, but would certainly take FOREVER. If you don't understand this, hit me up in the comments and I'll elaborate.
3D Scanner
- This is what I did, but DON'T PANIC! You don't need some fancy and expensive 3D scanner like the one I used, there is software available that uses a technique known as photogrammetry which compiles 2D photographs into a 3D mesh.

I am lucky enough to have access to a high end 3D scanner (shoutout to Jacobs Design Institute!) and so that's what I used. When using one of these bad boys, you may need to augment the surface finish to increase the fidelity of your scan. Also, depending on how yours works, you may need to add positioning dots to keep the computer oriented. Although I could write an entire Instructables on scanners alone, I'm going to assume that if you have access to a $40,000 3D scanner you know how to use it.

Step 2: Concept Design

This is where you will need to make some design choices about how you will access the bag. I chose to go with a snap fit lid. You could also go for something like a living hinge and an elastic band, Velcro, latch mechanism or anything else you can think of. When choosing this, make sure to consider what manufacturing methods you have available. Check out my concept drawings for some of the other ideas I had if you aren't happy with my choice.

Step 3: CAD

CADing with mesh is kind of a huge pain. In order to edit it you need to turn the history off, meaning you can't go back to undo things more than a few times with ctrl+z. Once again, this is pretty advanced CAD so I won't be going in depth. There are countless videos on the internet, and I would start with videos pertaining to mesh

I have a student license for Fusion 360 as well as all the other Autodesk products and also Solidworks and Ansys. Obviously I didn't model it in Ansys... I have too much self respect! I chose Fusion over Solidworks because I am more comfortable with mesh in Fusion. I ended up using Blender as well, but we'll get to that :)

Anyways, enough about me. I want to hear about you! Just kidding, back to me!

The basic design flow was to reduce the mesh to as few triangles/points as possible. The less you have the faster your computer will handle it, and the less likely it is to crash. Make sure to save frequently. A pro tip is to make copies at decisive forks as you can't go back that far with ctrl+z. After cleaning up the mesh, create a surface on either side of the frame where you want your bag. Then create a center plane between those two planes. Use your center plane to draw the main body of the frame bag, and then extrude it up to the planes on either side of the frame. Use the frame mesh to split the bag. Then delete the extra bodies this surface cut created. You should now have a box with curved chunks missing that fits your frame perfectly.

I then turned the geometry into a surface with no thickness. I took this surface into Blender, where I gave them both thickness. I made the box thick on the inside, and I made the lid thick on the outside. In CAD this would work, because they originate from the same surface the two pieces would have a perfect fit. However, we will be 3D printing and so a gap is necessary. Luckily, you can offset a surface before giving it thickness in blender. I gave it an offset of about 1mm and this worked beautifully.

I then brought my mesh file BACK into fusion 360 to make it watertight (you might not have to do this) and made some of the surfaces flat which helped with printing.

Step 4: Vacuum Forming (process)

Mold Design: When vacuum forming, it's important to make sure air can get sucked out of everything. To make this possible, I included holes that allowed the vacuum to reach into the concave features. Another thing to think about is draft angle. Since my part has a draft angle going the incorrect way, I had to split the mold into 3 parts that would allow the mold to be removed from the formed plastic.

Mold Production: Odds are you'll need to 3D print your molds, there really is no other way to make them besides additive manufacturing. Sure, you could probably use a 5-axis CNC with a tiny endmill, but why? I went with a really high infill of 50% and thick walls. This is important because your molds will get very hot when using them and you don't want them to deform.

Give the molds a light coat of mold release (self explanatory) and use double sided tape to space the molds off of the bed to improve suction.

The molds are pretty tall so get the plastic as hot as possible, turn the vacuum on, and pull it down over the mold. Let the plastic sheet cool, and pop the molds out. Then cut away the excess plastic and you've got your part! You will also need to add a hole or two for zipties to secure it to your frame. I did this with an exacto knife but I would recommend a Dremel.

Step 5: Vacuum Forming (conclusion)

I used the thinnest ABS available to me (1/16") and I think that a big pitfall of the vacuum forming process for me was a lack of thinner sheets. 1/16" ABS is just too thick to get into the clip mechanism recess. I think another issue is that the holes on the bed of my vacuum former are two far apart for such a small mold. One issue on the taller mold for the box was webbing, which I suspect could be cured with a thinner plastic but may be an unavoidable design flaw. It's hard to say without more testing. And maybe the biggest issue was a weak vacuum.

The molds held up really well and what did work fits REALLY well in my frame! I would love to try this again with thinner plastic on a smaller vacuum former with a stronger vacuum.

Step 6: Prototyping

I started with PLA for prototyping, as it's cheaper and I can print it at home. Once I had all the kinks worked out I printed the final part on a Markforged printer. It can print several things, but I chose chopped carbon fiber reinforced Nylon. This is sort of the default. I would have added continuous carbon fiber strands, but the parts are so thin that there isn't room.

If you look at the first picture you can see a gap between the lid and the box. Additionally, it doesn't rattle at all when you shake it. This means that the lid is slightly deformed from where it would "like" to be, which is the position it was printed in. What this means is that there is a constant pressure on the plastic bending it outward. I believe all materials creep, but plastic especially. With the lid being bent outwards, it will eventually deform to that position and lose it's snap. Therefore, it's really important that the lid fits without being deformed. If you see the second picture you can see what it should look like. It does rattle just a little bit, but it isn't noticeable when you're biking.

Step 7: 3d Printing

MY FILES

The Markforged slicer looks really cool, but the pictures don't reflect the orientation I ended up using. I changed the orientation to reduce the amount infill. I don't have much to say about printing on a Markforged; just like the Creaform scanner if you have one you probably know how to use it.

In hindsight I probably didn't need a brim on the 3D print. A raft or brim is useful while printing PLA or ABS because it's possible the part could pop off while printing. But on the Markforged the parts bond much better to the build plate.

Step 8: Conclusion

I am super happy with how this project turned out. I learned a lot about vacuum forming, 3D modeling with mesh and surfaces, 3D printing on the Markforged, and gained some invaluable design intuition.

I can tell that the PLA parts are not as strong as the Nylon/Carbon ones, but the PLA is actually a little more rigid. The rigidity gives it a nice snap when putting the lid on, but the Nylon/Carbon parts don't rattle.

I haven't used this on any epic rides yet, but I did do some riding on super rough streets, off curbs, some bunny hops, etc and the lid didn't come off!

If you plan on riding in the rain or letting it sit outside, I would recommend drilling two small holes in the bottom of the box to make sure water doesn't build up. Worst case scenario it would reach the zip tie level and pour out through the holes.

A special thanks to Adam and Chris at Jacobs hall who helped me with 3d scanning, vacuum forming, and 3D printing on the Markforged. Also to William and Adyson for editing this post.