Align the centerline of the measuring panel with the centerline of the plywood platform, and mark off you measurements.
Here, I'm marking directly on the plywood, but I decided to layout my shape on a piece of cardboard first. That way I was able to check the fit and make minor adjustments before cutting into the Baltic birch.
Connect the dots on the cardboard template and cut the side curves. Test fit the cardboard pattern, and when you're satisfied with the fit, transfer the outline to the plywood.
Cut to the inside of your line with a saber (jig) saw. Don't try to get the fit too close - it's better to cut a little small and leave a gap, than to be forced to keep whittling it down because you cut it too big.