PodDude, as I thought, there are plastic covers available that should meet that security need. I just wanted to be sure you didn't need a "Fort Knox" solution. Here are a couple of links:
http://www.rvplus.com/jr-products-hatch-water-locking-p-wht-a6112-a.html?gclid=CM2U2uy-0swCFQIOaQodg6IJOA
http://tweetys.com/valterra-a01-2002bkvp-fresh-water-inlet---lockable-gravity-hatch---black.aspx
To avoid cutting a hole in the wall, you might be able to cut off these doors everything except the cover and shim it out (to whatever extent needed) so there is no interference.
Also, one of the things I did on OurPod was replace all hatch cylinder locks with ones whose key matched my door key. This avoids the possibility that everyone and their brother has your common hatch key on their chain. See the products at:
http://www.rvlocksandmore.com/Global-Link-and-Fastec-FIC-Brand-RV-Locks-and-Keys/Security-upgrades/CLB-Cam-Lock-Barrel
Hope this helps!