30x45 picture frame in 6 parts (easily customizable to any size, using the OpenSCAD file). It is lightweight (about 125g for 30x45) but reasonably stiff. The separation between parts is noticeable from a close look, not obvious when looking at a picture of this size.

Creality

Ender 3

No

No

0.2mm

20%

Print 2 copies of each part i, l, l2 (note that l and l2 are symmetric but not identical) and 6 links. Assembly is obvious !

Dimensional accuracy of the printer may be something to consider given the dimensions and the tight tolerances (the glass is assumed to be no more that the specified dimensions, and is only covered on 2mm). Specifically, if printing any of the part diagonally, the angle between the x axis and the y axis must be really right. In case of inaccuracy, the dimensions of one of the part should be corrected after printing the two others to compensate.

Printing at 0.1 may be an option to really hide the layers on the segments that are printed vertically. A slight sanding of the surface that lays on the print bed can be useful to remove the glossy aspect if printing on a polished surface.