Since you are almost starting from scratch with making these bars again I think it would be advantageous to make same changes to the design to make them more functional and safer for the people buying them.
Increasing the angle so the harness straps lay on your shoulders properly when using this bar with stock seats would be one welcome and easily made modification to the current design. Adding small bars,tabs or hoops onto the bar to hold the harness straps in place would also be a very nice addition.
I want to point out that anyone that has one of these in their car should not let someone ride in the back seat, even with a seat belt on.