For short lengths a 5C spin index should work well.

The entire length of the finished part is octagonal. Yes, I
could make it out of round stock in this manner, but...

I was thinking he meant with a square collet. Actually, with a square
collet, you may not even need a spin index, just a square collet
block, and mount the collet in the block at 45 degrees...
--Glenn Lyford