Intent: To be a beginner (and intermediate guide) on making DIY cases. This guide is mainly designed for cases made with FFF/FDM printers. However, design technique/guideline can also apply to non-FFF printed cases where apply.
General design guidelines
DIY-battery Has most of the info, but TLDR: Best practice is to make 3D model of your battery pack, and then design case from there. Always leave space for BMS and other padding/dampening materials.
10%~20% of space should be more than enough, if you don't know what you're doing. FFF Printer users: Do not make "tall" cases that are printed bottom to top as a whole. account for tolerances. if you are printing cases that has fasters on it with no metal threads, pretend those fastener holes to be expendable. FFF plastic threads don't last long.
Limitation of FFF/FDM
Due to how the printer works (bottom to top), anything that are printed bottom-top that relies on any side with 3d printed material is prone to fracture.
Layer adhesion: Most people who use FDM printers as a hobbyist don't know the secrets of making strongest parts with FDM printers. and that is by not making parts that relies on layer adhesion strength.
Analogy: Think of a house with bricks, but no mortars/support/adhesive. you kick one brick; the whole wall loses support. The further down this case fractures, more likely the whole part will just rip apart. If you have to make a case that is "tall", pretend that you're banned from using infill, and print as slowly as possible with maximum heat possible for best adhesion.
Best printing/design practice (for FFF/FDM)
Generally, if you are printing a case, you're printing some sort of box. now, divide this box into sides (you'll get 6).
Now, model in a such way that these cases all have threads and will rely on fasteners to hold their weight. more fasteners, more weight that are evenly distributed.
Something like handles would most likely be printed in 1 or 2 parts: in this case, print the handle bottom to top sideways (so, tangent/perpendicular to the force being applied). Bottom to top would result in you holding the battery with handle that relies on layer adhesion.
Fractured handle while carrying = battery drops = potential fire hazard.
Case sides with multiple part
For example, 1 side of the case that is 2 parts (most likely split in 2 along the center).
This is bad practice because you've created a gap where water/moisture can seep through without having good way of sealing this gap. also, due to this gap, your case is structurally vulnerable, since there isn't anything holding on the side of case that is split.
So, if you are making a battery that is larger than your 3d printer's build area/volume, please reconsider.
Other methods of making case
1. Cut sheet metal, with FFF printed plastic or other forms of structural part for its exoskeleton/frame.
2. Plywood, but probably a bad idea if you're leaving case out in the weather. In this case, consider coating plywood so it doesn't rot.
3. SLS. Most likely inaccessible for majority of people reading this, but your college makerspace might have this. In case of SLS, the argument of layer adhesion strength would not apply, as this only applies to FDM. You can also make a custom order to companies that do printing service, but they often charge you a ton so this might not be acceptable for some.