I just followed a tip I remember from Dracula. It wasn't an oval I was working on, just straight lines but I remember Dracula said he uses it on his ovals. Blue painters tape works a treat because it sticks quite well, much better than contact paper. Don't bother trying regular masking tape, it's useless. For an oval I guess what you would need to do is stick overlapping strips of it together to create a sticky sheet, then cut out your oval and discard it, what's left behind you would lift and stick to the canvas, but I haven't tried it. Brilliant tip from wybl. You can paint the whole canvas with gesso, you can go a step further and add another layer inside of your oval after you have applied your mask and that helps to prevent your oil paint seeping under the edges, I've done that, it works a treat.