I got a nice steel shaft out of a straight-shaft gas weedeater. It's about 5/16 inch in diameter and about 4 feet long. All I'd have to do to make an excellent probe is to get a 1 foot long section of 3/4 inch water pipe for the handle. Drill a hole in one side of the water pipe at the center point that is just large enough to insert the end of the shaft. Weld the two together then use a grinder to sharpen the other end.
Oh yeah. That steel shaft was INSIDE of the weedeater shaft. Unbolt the head section and pull out that solid shaft. I found mine in the local landfill.