I use a plywood jig. The jig has to be the thickness of your piece. One side of my jig has bigger dado spacing for a bigger spacing on the trivet. The size of the trivet has to be square and fits in the middle of the jig but so the corners of the trivet are facing the flat sides of the plywood jig. The trivet has to be centered in the jig. I start the dado 1/4 from the diagonal of the trivet and make the first cut.(dado bade width is 3/8") the spacing is also 3/8 and just continue on till the end. But when you start the bottom of the trivet you have to turn the trivet one quarter of a turn so the dado isn't going in the line of the other one. I sprayed the finish on.
You could probably make a jig for a round one that would work as long as after the first dado you put a strip of wood (identical dimensions to the dado and the same length as the jig) into the dado to act as a "key" so that the round wouldn't rotate between cuts.