Thanks guys!
Pete, I have one of those too!
I took another look at what Bob had done in his article. I copied his template and enlarged it a bit. But the flat sides of his template form a convex curve when placed on the tubing.
I drew a 12" circle on a piece of card stock, cut it out, then traced it onto his template (so the straight line looks like a concave curve on the template).
It worked, making the concave curve a straight line when looked at from the side. Only problem was the bottom of the curve was higher (from the bottom of the tubing) than what I need.
The solution (I think) is to change the 12" diameter circle to something like 30" diameter, giving it more shallow curve. When placed on the tubing the template should appear more flat when looking from the side.
Big Jim, Not only did I take 3 years of mechanical drawing, I went thru an 4 year apprenticeship at a machine shop then worked another 4 years. Of course that was from 1969-1976 so things are a tad fuzzy, but are coming back . Wish I still had my Machinery's Handbook, they're going for big bucks these days.
I could make the cut using Bob's template as is, then whittle the sides down until they mated with the coal bunker. Taking away material is easier than adding it back 
