I have not used an Ortlieb handlebar bag. Mine is a cheaper one that has fairly flexible plastic side pieces, that flexed way too much.

I tried using two 3/4 X 1/8 inch aluminum bars as an additional support, they bent too easily. I then tried two 1.5 X 1/8 inch aluminum bars, it has been strong enough to work very well. They are between the bracket and the bag, and bent at the bottom to provide support under it.



I stretched some inner tube rubber sleeves over the ends to provide a surface that does not abrade a hole in the bottom of the bag.

My 3/4 inch wide bars eventually cracked at the bend at the bottom corner, when I bent my 1.5 inch wide pieces I used a wider radius to the bend so that the stress of bending was less concentrated in one spot which I am sure helped.