So, do you think there is room to seal-a-meal the rescue me and then place it in the camera case...as added insurance against flooding?
That's a good idea. If you want to do that, you may want to first put the PLB1 inside its float pouch, then vacuum sealed it with the seal-a-meal, before putting them in a waterproof case. My old underwater camera case would be too small for that. You would need the waterproof case that looks like a cylinder, like Ken T uses, as shown above. So when you pull it out of the case & you accidentally drop it in the sea, it will stay afloat, it won't sink into the bottom of the sea.
In my case to fit it inside a box-form underwater camera case, I have to attach the float pouch to the PLB1 with a string & place the pouch next to the PLB1 in the camera case. I guess you can try to seal-a-meal the float pouch & the PLB1 next to each other to make them like a box shape vacuum sealed unit rather than to look like a ball or a cylinder shape vacuum sealed unit.
On the other hand, the PLB1 is waterproof up to 50' (25 psig). If the camera case leaks at 100' depth, as long as the leak is slow enough for an hour dive to not filling up the case completely to pressure up the inside case up to 25 psig, I won't worry too much about vacuum seal it.
The nice thing about the plastic underwater camera case, you can actually see if it leaks. If it leaks a little bit, as long as there are plenty of air inside, the air is compressible, so the pressure inside the case will be lower than the outside water pressure. If you dive at 100' deep & see a little bit water leak inside the case, I won't worry too much about flooding the PLB1. When you are done with the dive, just drain the water out & fix the leak with new oring or may be it's just a dog hair stuck on the o-ring seal that you can remove it & reapply the oring surface with silicon grease.