I think I bought these inexpensive T-handle Allen wrenches more than 25 years ago. Over time the some of the plastic handles developed fine fractures near where the hex wrench and handle join. I decided to try repairing the fractured plastic handles with stainless steel ferrules and these will also improve the strength of the handles. You can see the fracture line in the photo showing the handle diameter being machined on the lathe.
Rather than first drill and then bore the stainless steel T-303 rod, I decided to first machine the plastic handles to a diameter 0.002" larger than the next nearest fractional drill. This allows direct drilling the ferrule hole diameter to fit for a press fit and avoid any tedious boring on the lathe. Sometimes the drills can slightly over-size the hole IDs so the 0.002" larger plastic handle diameter usually ends up with a 0.001" difference in diameters and an easy press fit. The ferrules were press fit using an arbor 1 ton press.