I know this is a year old thread, but I spent ages trying to figure out how to do this on my '00 Outback so I thought I'd share my experiences!
I have my Cibie Oscars mounted on one of those licence plate bar things, but as you mentioned on our cars the plate doesn't screw into anything solid so with lights they'd bounce all over the place.
The solution is this.
I got two metal spacers made up, they're basically a hollow cube with a hole through the middle, I then cut out two square holes the same size as the spacers in the plastic bumper (still small enough that they'd be covered by the licence plate if I ever take the lights off) with a pocket knife and put the metal spacers in. Then drilled a hole into the chassis using the spacers as a template. Then put a big long bolt through each of them and the light bar. Gives you a solid, flat mounting point for the bar and no bouncy lights.
That might have been a bit confusing so please pm me if you want more details