first i made an exact clone of the base template so that I would not screw up the original.
I took the copy, and with router bit and spacer combos, i found a combination between a spacer and bit that ...no....actually I found a rounding bit that had a flat portion to it at the end, where the flat portion was 1/4" bigger than the bearing it had. I adjusted so that the bearing rode along the very bottom of the template, it cut the round in but the top area was flat and 1/4" thinner all the way around. I then used a straight bit with a top bearing and trimmed off the rounded part.
Just figure out a spacer and bit combo, or a bearing and bit combo, that will allow you to cut into the cloned template and reduce its size by whatever you want your sidewall thickness to be, dont forget to account for edge thickness. Then I would say just figure out the Puzzle piece part and jigsaw it with a small scrolling like blade, that part will probably have to be freehanded and be more difficult. Maybe cut one side of it out on a piece of plywood or something, then sand it smooth, then clamp it down and router it out with a small diameter bit and bearing. Then flip it over and do the other side exact same shape.
to construct a core template using the larger base template?