I was in need of a small shelf by the entry for my car keys, so decided to give it a go.  Here's what I came up with.  Pretty simple, but gets the job done.  

Overall dimensions are 200mm x 100mm x 90mm or approx. 8in wide x 4 in deep x 3.5in tall.

It is designed to use 2 medium sized Command picture hanging strips, velcro-style.  Similar to this:  https://a.co/d/07B24eQP

Settings for my print:

• PLA

• .20mm layer height

• 10% Infill

• Supports - I chose to have them on, but you may be able to get away with them off.