A friend has built a wooden arm rest to replace the malstick. He likes to work on a flat surface rather than at an easel. So in order to protect his oil painting while he is working on a particular section he places this arm rest across the canvas.
The arm rest is made of two blocks of wood held together by a long strip of wood a bit bigger than the size of the canvas. The blocks would be resting either side of the canvas leaving enough space between the strip of wood and the painting.
Make sure the strip of wood is sturdy and does not bend right down to the painting when your hand is resting on it.
I suppose it might be useful for other media such as watercolours, pastels, and acrylics too.
Please let me know if you want a drawing of the arm rest.