Trying for a Diamond Goal tommorow - water ballast question

The forecast tommorow looks fantastic down here in Memphis! Looks
like perfect conditions to load up on water.

I have water bags in my Std. Cirrus that I have never used. Looking
at the wingroot, there is a hole that the bags are bolted to. These
bolts are easy to unscrew. This hole is appx. 1 and 3/4 inch
diameter. There is another hole about 3 inches in diameter that has a
clear plastic peice glued over it. My thought is that the water bags
are pushed into the wing through the smaller hole, then get pulled out
through the bigger hole. I don't want to just cut the clear plastic
peice off unless somebody can verify that it is indeed what the
plastic peice if for, or explain another way to test the bags for a
leak other than just pulling the bag completely out for a test.

My guess is that the clear plastic peice is for sealing. The cirrus
wasn't designed with bags, it was designed with fiberglass water
tanks. The water ballast bags are an aftermarket addition.

Thanks in advance,
Scott

Anyone have any experience?