Pat if you make one to to your satisfaction with the straight sides and curved corners you can make another smaller one by hand with more than reasonable accuracy. If you take a thin board and put a peg through it that will follow the outline of the original and drill a hole for a pencil where you want the outline of the smaller jig to lie then you merely have to trace from the larger to make the smaller one. Of course you want to stay perpendicular to the edges and the curves but you would have to be off vertical to them by at least 10 degrees before it starts to become noticeable, something that is easily acheivable by eye.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.