On effigies, 14th century heater shields usually look to have elaborate, 3-d designs on the front, made from molded leather. (Like the Black Prince's shield.)
Would it still be historical to simply lay down a layer of canvas and paint it?

For what type of person? There's a big difference between the elaborate knight shields used in tournaments and the shields used by soldiers and man-at-arms in wars. The former is a status object as well as a piece of equipment. The latter is a dispensable piece or warfare gear.

These shields come in all kinds of varieties. The 3D versions are by no means universal. Most of the extant shields have a parchment cover. canvas has much the same effect.