Your best bet might be to bend up something yourself from a bit of scrap, and use that as a model.
Then go scrounging around the auto graveyards looking for something a similar shape.
That will be a lot easier than making and comparing a bunch of inch measurements with every bar you see.
If you can find something off a reasonably late model car that will fit, you may be able to buy aftermarket bars in a range of different stiffness to play around with.
You might be surprised what you can come up with. Something perfect is already out there, you just need to find it......