Hi Gene
This should work.
Make it with 3/4" plywood or MDF stock with 2 x 4 x 4" long clamp blocks.
Cut a slot in one part for a guide bushing to run in, clamp it to the pole,then drop in a plunge router in with a 1/2" carb bit or a flat bottom bit and put a flat spot on the column.
Note***most big colums ,4 1/2" or bigger, are NOT made from one peace of stock, the norn is 4 or more parts, the easy way to find out is to drill a small hole in the column then stick a small wire in the hole.
This will tell you not to cut to much off the column.
Bj
You may say that's alot of work just to put a flat spot on a column, you don't get a 2nd chance at this one
and I'm sure you don't want to replace the column because you didn't go the extra mile to get it right the 1st. time. 
I'm not sure what you can use the jig for once you have the job done maybe remove the bottom part and use it to make a flag pole
This should work.
Make it with 3/4" plywood or MDF stock with 2 x 4 x 4" long clamp blocks.
Cut a slot in one part for a guide bushing to run in, clamp it to the pole,then drop in a plunge router in with a 1/2" carb bit or a flat bottom bit and put a flat spot on the column.
Note***most big colums ,4 1/2" or bigger, are NOT made from one peace of stock, the norn is 4 or more parts, the easy way to find out is to drill a small hole in the column then stick a small wire in the hole.
This will tell you not to cut to much off the column.
Bj
You may say that's alot of work just to put a flat spot on a column, you don't get a 2nd chance at this one
I'm not sure what you can use the jig for once you have the job done maybe remove the bottom part and use it to make a flag pole