waterproofing rear barn doors

Hey so its been a while since ive posted...i was the guy who got a brand new haulmark 6x12 that was rusted out..all went well and they built and delivered me a brand new trailer and took the rusty one away..this one was built way better with no rust..anyway i have been very busy working two jobs all year long which is why it took me so long to start on the trailer..ive been steadily working the past couple months and the trailer is 90% done...i will start a build thread soon with plenty of pics...anyway one thing that has me a little perplexed..i built a false wall in the rear behind my barn doors and use it to slide an air conditioner in and out..works great but not sure what to do if its raining...the barn door is open while using the ac leaving the inside wood of the door as well as the false wall exposed to the elements....do i rig some kind of tarp over the rear? Do i waterproof the false wall and the inside of the barn door? What can i waterproof it with? Im sure some of you have had the same issue..would love to hear how u solved it