I once needed to mount some extra stuff too and did not like what was commercially available. I took a piece of PVC pipe and cut it long enough to fit between the forward position of my drop bars, just beneath where the brake levers were. I cut slots in each end and ran a hose clamp through each end and secured those to the bars. This made an accessory bar that was about 42 cm wide. I could mount a lot of accessories!! LOL. It worked well and I could place a handlebar bag between the bar and handlebar. Unfortunately I never took any pics of that set up.

The idea is hardly original, maybe with some research you might can find someone else who took pics and posted them.