I recently had to machine an aluminum mold cavity.
7 inches deep. With 5 degree wall draft and a 60 thou radius going all the way down. Roughing was not an issue, but for semi-finishing and finishing i had to manufacture these two extension holders.
Both tools have runout of less than 0.001
The one for bigger 3/8 tapered ballnose cutter is shrink fit- i mounted it using torch.
The smaller tool is a 3/32 tapered ballnose cutter from Harvey Tool.
I could not bore to correct size, and had to ream right on.
The tool is mounted with a set-screw from both sides to prevent deflection caused by unequal clamping pressure.