I've never seen any difference in direction for routing slots, but if you're routing an edge, there is.
There was a excellent version of a radius circle-cutter jig like you're wanting in the Woodsmith Router Workshop book. It was made out of plywood, and all of it 3/4 stock if I remember, but that can be changed. The only plastic they used was the knobs for adjustment.
If you're still intending on using the Rotozip, then you could make a clamp with the plywood to clamp around the round nose of the router, and hold it vertical.
Since that's 1/8" plywood, you might be better using a 1/16" bit. I know Freud makes one. Also, you might have to try a spiral bit, I'm not sure due to the thinness of the plywood.