I'll be describing how I made a vertical rack out of corrugated cardboard, to house my handphone and battery-holder. I'm just going to give a image sequence of the process. The method should be pretty self-explanatory. The end result looks like this:

Cardboard rack, housing a smartphone and a battery-holder
Ingredients
  • Corrugated cardboard
  • Penknife
  • Ruler
  • Paper glue

Steps
1) Cut out the walls with tabs attached. The walls are the cardboard pieces that hug the electronics snugly. The tabs jut out below the walls, and are there to keep the walls fixed to the base. Dimensions are roughly estimated.
Here I have two "walls" for the smartphone and two for the battery-holder.

2) Cut out slots for the tabs. Cut out as many slots as you have tabs. I have 12 tabs, so that's 12 slots for me.

Slots on my base plate.
Close-up of the slots. It takes a bit of knife-fu to make clean cuts.
3) Affix walls to slots. This is basically pushing the tabs into the slots. As an option, you may apply some paper glue, sparingly, to the tabs. But if the slots are well-sized, glue isn't necessarily.

End result of affixing all the walls. The four tabs at the far end are a little too short.
This is how to apply glue to tabs (optional).
4) Split the tabs...

Split the tabs right down the middle, and flatten them out.
5) ... and glue them to the base. This gluing part is challenging because the tabs tend to spring away from the base. What you can do is press the tabs with your fingers until the glue dries, or if you're impatient just use tape instead. 

After gluing.

Conclusion

Here's the completed rack, empty.

Viola.
And here is the rack in its natural habitat. You can see why it's good to have the rack vertical. There isn't space to put my smartphone or battery holder, unless I stack things upwards, which is a pain.

Bare top half of my robot - shown here are the Raspberry Pi, Arduino Uno with motor shield, breadboard, and cardboard rack. Wiring and other electronics sold separately.

 I hope that was useful! Have a good day!