that migth be a good use for a optical center punch (but mr pete on youtube reviewed them, and its not like optical is gonna get you perfect (he got 0.005 accuracy between pins made with optical center punch), but it is ALOT cheaper and way more portable.
I do suppose it would be possible to put a lathe chuck on a drill press if you get the right taper for slow marking with sand paper if you want to go to 4 inches, maybe.
As for the channel, I think its impressive that he actually got that stuff to work. He also does a battery powered angle grinder this way, and actually cuts some square tube with it... yes.. its developing world quality stuff but its interesting, it looks quite professional if you believe it can be made that way with the materials he had.. modern day "african queen" (if you don't know, its a movie with people stuck in the african wilderness for a good portion of it)
1 hour 3 minutes in the macguyver starts