Do you have enough room on the steer tube for another stem? If so, install it right under the top stem but point it down. Use a shortened flat bar for the bag to mount to. This will lower the bag and leave you room for the cross levers, and other bar mounted accessories.
Or maybe something like this from Siren.
I've done the double stem trick for getting an Ortlieb bar bag lower. Works great. Cool idea to use it for mounting a harness too. That clamp from Siren looks interesting too.