At a 1/2" off I would be tempted to plane all four sides to fit, rather than sawing two of them. A circular saw can make a mess of things in a hurry.

* Remember to make it slightly smaller than the opening, or it will jam when it swells in the rain (like mine does).

* Paint or varnish the edges.

I have a circular saw, and a 6 foot straight edge. And, likely the clamps to hold it all in place.

Whether I can quickly lay hands on them, in the half-unpacked shop, is another question.

I have no idea where my planes are, and they are not in great condition - but Corwyn's right about doing it just about to size, and then planing to fit, and finishing the edge.

He's way better at this stuff than I am.

Using a plane properly takes skill. If you lean left or right, you turn the edge of the door from a right-angle, to a sharper angle. That may not be what you want. I wouldn't be so nervous doing that when shaving off high spots, but I wouldn't want to remove an inch or so of door that way.

I'm not that good. Planes do take skill. I think I COULD do it, but I would allow as how it is odds-making for me, not a sure bet.

A circular saw will get you close, and preserve the right angle, at least, it will do so more easily. (You CAN tip a circular saw to the left or right.)

My table saw is still in parts, and not set up, or I would recommend that. :-) Place the whole thing on a plywood board, run it through the table saw...