Clown, the Heli flybar would be a great arbor.
You have to center the cone, best you can.
I have found, that even with a slightly bent arbor, you can still get a balance.....you rotate the arbor 180 degrees, and recheck. If the results are the same, you should be good to go.
(I learned this, called "Compensate" when I learned to do 4-wheel computer alignment on cars)
By flipping the arbor 180 and rechecking, your cancelling out any discrepancy.
You can make up custom centering cones, using fuel tubing, Sullivan rubber nuts (from the Gold-N-Rods) brass tubing, masking tape rolled around and around, wheel collars, or any combination of above.
My thread shows, me using a long Higley drill bit, a wheel collar to retain the stock centering cone on the pointy side of the spinner, and brass tubing on the back plate side.
Self stick Lead Tape (sold for balancing golf clubs.....otherwise, it's a hard item to locate)
After cutting squares of lead tape, the spinner cone comes to rest in any 360 degree location. Rotating the drill bit arbor 180 degrees, yields same results.