Main Motorcycle: '99 Thunderbird Sport
Join Date: Jul 2013
Location: West London
Other Motorcycle: '99 Sprint Executive
Extra Motorcycle: '61 Bonneville
The idea that hollow tube is stronger than bar is a myth. It is stronger than bar weight for weight, yes, but hollow tube that is lighter and stronger is unfortunately thicker/wider - larger diameter. So there'd be more possibility of getting the holes too close. And tube in this application, where you're squeezing it with a point load from the side plates, doesn't look good.
By far the easiest is to get a length of drill rod - O1 steel - silver steel, which is pretty tough in the unhardened state. Don't harden it - just use it as it comes. For really hard and tough you'd look for a length of chrome-moly or chrome-vanadium - that's the stuff they make socket set t-bars out of. That's getting a bit pricey though.