You have a narrow width between your wall and your door, and you don't want to loose this space ? 

I couldn't find any shoes holder online to buy or 3d print that takes less than 15cm width.

Here is my solution for all these constraint :

• 15cm width
• Fit A1 mini
• Minimal use of material (<80g per floor)
• Easy assembly
• No fixing/fastening on the wall
• Easy to grab and put back shoes
• Use with sneakers and other low-height shoes.

It fits behind a door with narrow space behind by putting only one shoe per ‘floor’.

Material needed per floor

x2 M3 screws 8mm long (if you don't have 8mm long, you can take longer)

x2 M3 nuts

Printing settings

I've printed it with A1 mini, PLA, .4mm diameter nozzle. You can reduce the infill to 4-5%, I've used honeycomb. You should use 'Z Hop when retract' in the filament settings, without it the print might fail near the end, when it is starting to make the last bridge.

Each floor takes 3h of printing, so it should takes 30h to make the same 10 floors shoes holder.

Assembly

There are two configurations, named O and X.  It's better to alternate between O and X, especially if you have more than a few floors, to avoid an offset and having an un-straight tower :

To assemble, just clip the elements (check the circled slots so that it fits) and then add nut and screw them together.

Recommendations/warnings

• It also work without these screws, but it can easily disassemble if someone pull it upwards. So for real long term usage, I highly recommend them, but if you just want to test the idea it's not needed.
• The elements are somewhat fragile if they hit the ground and might break if they fall.
• The bridges are long and depending on your printer and filament, it may not look nice visually, but it doesn't impact the structural strength.
• Only tried up to 10 floors/5 pairs of shoes.
• I've added a wood plank for a little more stability at the bottom.