How do you cut birds mouths?

I have to cut 40 birds mouths, the short side is 2" and the long side is 4" (it's for a gambrel shed). I was able to cut the 2" part using my radial arm saw and tilted at an angle and the pieces standing up (I could cut 4 at a time). The long side has me baffled, too much depth for my radial arm saw and I can't rotate it enough to get the angle.

I started with an orbital jig saw but it's so slow and doesn't seem to be the tool for the job. How do you do it?

Thanks