I designed this little Shelf in Solidworks to have a easy way to add little shelves everywhere to display cool prints or photos and such stuff.
I put it on the wall with some double sided Tape and it holds very well for the small stuff thats supposed to stand on there.
----- UPDATE ------
I added a version with holes for mounting the shelf with screws to the wall!
And i can tell you that they hold very well on the Wall with Tesa Powerstrips Large :-)
Printed with 0.2 layerheight
10% to 20% of infill
Standing on the side with a nice big enough Brim to support it.
If 240mm is to tall for your printer, just scale it down on that axis.
If you have very flat tape you should print the flat version. The normal version has a little cutout for my thick tape.