For anyone who is interested and simply to finalize this thread:
The GM part number for the lockable fuel-cap is #10326376. It is being sold in similiar form for the H2 (same part number). It is pretty small (because it usually goes behind the chrome gas doors) and doesn't look really good, but get's the job done.
Price should be around 50 USD
Thanks again to H3Lady for passing on the GM part number.