Many years ago I could not afford a coring system so I made one.
Took an ordinary steel bar about 1 1/4 by 1/4" and bent into a curve similar to the McNaughton ones. for a cutting tip I split the tip of it and spread it into a Y and sharpened them. Did not have a restraining system so I mounted it into a piece of pipe about four feet long and just eyeballed it. Sort of scary but it worked surprisingly well. At least I did not die.