For some given gouache painting. As I value them I designed this simple frame.
It used 8 magnets.

Parametric: width and height of both solid are linked by constraints in the first sketch by formula.

The original model is tuned for a 140*105mm paint, 2mm all around to hold the paint.

External dimensions = paint dimensions + 20mm

Internal dimension = paint dimensions - 4mm

Intermediate dimension = paint dimensions + 1mm

The frame is made of two parts holding together thanks to 8 magnet: diameter 6mm, height 2mm. 

• First put / glue the magnet in the facing/closing frame. 
• Then put in place the back of the frame, put a drop of glue, let the magnet fall in the hole and press a bit if needed. Like this don't need to worry about polarity. 
• Just be careful to not put too much glue. One you don't want to splash some, two if two much is applied on the facing frame, the magnet will protrude from the frame and leaving a gap between the halves at the end.

I designed my frame to be attached to the wall using only one nail (see picture): 6mm, 4mm rode, |~2mm high

As  always with my model you are free to modify it as I supply the FreeCAD file.

The frame by default is 125*160*(5+3)mm.

I printed in PLA+ gold, 3 contours, nozzle 0,4mm, 5 layers top and bottom, 20% infill honeycomb, no support, no skirt on textured plate.