That’d a really good task to add some accuracy into.
But in those sizes, and tapered, it’s a big ask.
However, it’s an everyday requirement when making, say, fly fishing rods, so why not adapt that type of solution?
First off, find or allocate a No60 ½ block plane, look for a decent old one, made by Stanley or Record or another good maker. If you have milling machine, mill a lengthwise V groove the length of the sole using a cutter canted 45 degrees or your choice of alternative, such as mounting the plane on the crossslide and the cutter in the lathe chuck.
You now have a plane which will self guide itself down the length of a spar producing an even chamfer. By starting with a square section you can easily make it octagonal in 4 passes.
Now we deal with the taper.
Imagine a long open topped box, with the sides adjustable so that they can move to be narrower at one end than the other. That’s what we need. The top, inner corners, are chamfered enough to allow a spar blank to sit there. A stop is an across the far end.
Place your blank in the adjustable groove, and open one end enough to let the spar sit down at its narrow end.
Run your plane along it. It will stop cutting when it has done enough. Turn the spar and repeat until happy.
Here, I’ll try sketching it.
View attachment 520889
The jig portion is held in a vice on the workbench. Adjust with shims until the degree of spar material you need to remove is showing. then use your grooved plane to remove the excess. Because the spar is supported along its full length, it should be distortion free. Your plane blade will be about surgically sharp, using the ScarSharp method or whatever, and if set for a light cut, should manage against any grain run out.
I have a device of this type on a much larger scale, and this is on the list of items to make once the masts are in place.
Apologies for the poor quality sketch. I'm around for further questions.
Jim