Worktable drawn in FreeCAD and built

My father painted many paintings, which he stored in a drawer. I thought it would be nice to frame some of them and put them for display on the wall so people could see them. So I started looking for frames, but I could not find any frame that would fit. The problem is that my father would just paint on a random piece of carton, not a standard size that would fit any standard frame for sale and none the same size.