Brining brisket to make corned beef; I saw a small bit was poking out of the top, so I put in a sealed gallon bag of saltwater, is this okay?

I figured I could just take up some more space in the vessel I am brining with, and used water in a bag to displace enough volume to push up the brine to cover the bit of exposed meat. I also thought that if the brine or water permeated the bag, it would dilute it, which would be bad, so I added in salt to the water in case it leaks, and then double bagged.

https://imgur.com/Ja07Q0p

Is this okay? They are gallon freezer bags which I feel have to be robust enough to hold water in/out for 10 days. The other thought I had were to find a rock, but then I'd need to wash the rock and scrub it to make sure I wasn't introducing anything bad that would mess up my brine.