homemade mushroom adaptors

[onedayTM]

Hi guys,
just wandering if anyone has made the adaptors as per the pdf file posted awhile ago, if so do you need to machine top as shown with the profile. the shape looks difficult to machine. I noticed that the rally design one albeit not offset it is flat below the tapered hole. Does this make sense to you, also what steel should be used for this.
cheers,
tony.