Question on using "Great Stuff" foam sealant

I've used that in various places, home and RV, to closed holes. In the RV, I used it to seal the hole where the water plumbing comes through the floor. Put some newspaper down on basement carpet else you'll never the the stuff out of the carpet. It does tend to go everywhere.

Like you I had a gap in the sheet metal under the shower. The problem was the the shower is over the wheel well and the road grime and water made a nice mess. I screwed down the sheet metal and then used great stuff to fill in the gap on top.

You can get a can with a trigger type applicator tube which does stop, but to be honest if you don't use the whole can, just throw it out. You'll never be able to clean the stuff out of the tube so you can use it again.