The jigsaw and the long blade is the best solution so far...
I can't see that working. Even if it is possible to cut a 2-3" diam half circle with one of those long blades (which I think is unlikely), the combination of tooth-set, blade-width and desired radius will lead the bottom end of the blade to drift, causing the cut to be conical, not cylindrical. Hopefully, for those who try the jigsaw approach, I'm wrong.
A hole saw has a built in depth stop, the top of the saw base. It will only cut through about 2" of material,... then what?
1) mark the 6x6 stock
2) use the hole saw to a depth of 1" or whatever, 4 cuts, not 8
3) use the circ saw freehand to run a bunch of kerfs
4) break out the bulk of the fins
5) use the hole saw to finish up boring the full depth
Hogging out the waste means the center drill bit of the hole saw will not have any wood to cut into. Hence the preliminary starter holes in step 2. Then in step 5, the hole saw is constrained not by the center drill bit, but by partial-depth starter holes cut in step 2. The point of this method is to avoid having to extricate a broken off core plugs from inside the hole saw.