Step 1: Make the rim
So, I took a 4' diameter tabletop that was kicking around, and drilled a 1/2" hole near the side, and tied a bit of rope around it, as shown. Next, I laid my channel iron face down (the open side - the side you want facing outwards) down on the ground, put the table on top, looped the rope over the end of the iron (of which I had about 16'), and began to pull back as shown. This little piece of aluminum I'm using in the picture is obviously much easier to bend than the channel iron; be strong!
The steel will spring back a bit, so on a 4' table, I got about a 6' circle of steel. I stretched it down to be just under 5', cut and welded.