So for just a few bucks at the hardware store, I made this super simple circle cut guide for my jigsaw with a range of 9"-69" diameter cuts. While I probably won't need to cut a full circle at 69", I may be able to use the curve of a 34.5" radius to contour a cowling or fuselage side. I bought a 3' long piece of 1/2"x3/4"x1/8" aluminum angle, and cut a notch out of the 3/4" leg wide enough to securely fit my jigsaw. I re-attached the slug I cut out to allow fitting of the jigsaw to the 1/2" leg with super glue to serve as a rest stop for the saw, allowing it to be removed and re-installed in the same place to increase repeatability. Using a speed square, I marked a center line on the underside the entire length of the 1/2" leg, then measured from the blade to the center line making a mark every half inch. Next was center-punching and drilling down the length giving me an anchor point for radii from 4.5"-34.5". I marked all the whole inch increments in blue, and the half inch increments in red. I put a strip of white fabric tape on the inside of the 3/4" leg and wrote the whole numbers in blue. While it isn't fancy, it's easy to use and works well. Small circles are a bit cumbersome due to the size of the jig, but I have another smaller circle cutter for 10" and below. For thin materials, A block of 2x2 in my Black and Decker Workmate clamp table allows me to pin through the thinner ply. If I leave the blade close to the edge and pre-cut the size close before using the circle jig, I can fairly easily rotate the piece into the saw blade and keep the saw stationary.