Added thoughts...

Size of block = size of drill x 2-ish need enough metal for the locking bolt to remain rigid

If drill shaft < drill size (as it might be if its a big drill) then drill out the hole shaft sized first then use the proper drill to bore down 10-15mm to provide a locating seat... then remove drill etc as before....

Ally instead of MS... can't see why not, maybe go for a bit more 'meat' on it. Only issue might be getting a good lock on the drill... if you never want to use the drill again without some effort you could try loctite in the hole!

thanks for the +ve feedback