Simple and easy, and you already have the idea. A very thin piece of REGULAR plywood works great. There really isn't any really thin LitePly. And LitePly has very little ability to stand crushing, even the thick stuff.

I recently had a KYOSHO P40 Warhawk flutter the stab/elevator. The stab failed on the LE on both sides and the elevator horn had no problem wrecking the thin elevator balsa. You couldn't see what grade of balsa was in the elevator or the LE under the covering. And darned if you couldn't feel some really good cross braces, so this flutter was a real surprise. Both places had wood that any experienced builder would have rejected. Unfortunately, the ARF factory workers aren't experienced builders or they would have rejected that stuff. And I paid for it. I got off the throttle immediately after hearing the flutter but the elevator would only respond about 5degrees but only at full up or full down. And I was over a soybean field. And managed to control the "landing". So damage wasn't too bad.

The first picture shows where I've stripped the covering for a 1/16" Airply plate. The plate got some lightening holes in it, but not under or near where the horn went. With punk wood you need to get lots of area covered. But you don't want weight, because that actually contributes to flutter of a surface. The 2nd picture shows where I added some diagonal bracing in the stab. I did all the work from the underside so I wouldn't have to screw around trying to match the covering colors. That'd been hard, since the colors had faded like crazy from the sun.