I am only posting this for future reference, the OP has ordered a solution.
The Nitto brand one shown by Saddlesores in his post above mine, use something like that as an example to make one. Or, buy that one.
Years ago I used that older type of handlebar bag, but the steel wire support for it did not fit on my threadless stem, so I made my own support. I think it was 1/4 inch steel, not even sure if I still have it somewhere as that was a decade and a half ago. I sprayed it black but some of the paint did not bond to the galvanize coating.
More recently, I used a piece of 5/16 inch diameter aluminum rod to make a support for my small Carradice bag to hang on the back of my road bike. I used a stem with appropriate shim and wood dowel sprayed black to support the DIY support. Also used some plastic tubing over the aluminum.
My point is that if you have access to a big vice and some tools you can use to bend some steel or aluminum rod, you can make a support pretty easily. It may take a few hours, you have to be careful to plan out what you are doing first and be methodical in the process.
I am not saying put it behind your saddle like I did, I am only showing that as an example.