Jorge:
Don't know if you got my direct reply (and pictures) to your question. If you mount directly behind the baggage bulkhead, there's already a "doubler" in place. Just make certain if you use this, you also drill the antenna hole so there is sufficient clearance on the back side for the lock washer and nut. This location will allow you to use the vertical brace as a mounting surface for your coax.
If you don't use this location, I agree that you absolutely need a doubler under the rear skin. The loading on an external antenna coupled with the thin gage of the aft skin can lead to significant flexing. Plan to rivet the doubler in place. Make sure all corners have a very large radius.
Terry