Here is a few designs I came up with and been using for 6 months or so.  Prints out of PLA and can easily support up to 10 pounds when mounted into a stud like it's intended.  Easy to level and mount with 2 screws.  I would recommend a minimum of a 2in long screw, 2 1/2in would be better when going into a stud.  You can use a drywall screw if you prefer, a wood deck screw would also work well.  These are a bit stronger.   Be careful and don't over tighten it.  Then slide your shelf down on to the mount you installed on the wall.  That simple!  

You can always paint your shelf to match your wall colors too!  A little bit of sanding on your shelf and apply your paint.  

If your wall has a lot of texture you might want to install a washer as a spacer behind the stud mount to space it out just enough so your shelf can slide down with ease.  

Hopefully you find this useful for little items or even small plants or other items.  

Printing large items like this.  I hightly recommend keeping your print door closed on an enclosed printer.  If printing on A1 make sure your bed temperature is up above 60 C.   I have no idea how this will work on the A1.  Any feedback would be nice, but remember bed temperature is very important to prevent any warping on the edges of the square shelf.