Instead of going to a specialty metal store, I'd just head to the local Menards, Home Depot, Lowes.
You will find a variety of pipes there. I'd start by looking at conduit. Since you are going to have to paint it, you could even look at the plastic pipe. Head to the aisle that has tape measures & grab one off the shelf to measure the outside diameter. Since the repair will be covered up, you only have to be close. If you look around the house, you probably have something you could used for the piece inside. (Obviously, measure the outside diameter of the pipe before you go).
If there is a flea market near you, grab a tape measure & go. You might even find a Weber while there. Check out anything pipe-like while there. Perhaps an old canister vacuum metal "wand" might be the right size.
If you make it up to Chicago, you could measure your u-shaped piece & I'll compare it with the u-shaped piece they used on the non-performer one that has a table. I believe I have one in good shape, while the other u-tube is rusted, but it probably isn't worth shipping. If you are coming for the meetup & are not in a rush, I'll save it for you (assuming my memory is correct & one of them is ok).