What size dog?
We picked up a plastic tub meant for storing stuff under you bed
. The sides are about 6" tall, 2'x3' and put a piece of carpet in it. It sat in the cockpit
It took a couple weeks to get the idea thru but then it was a great option. Of course as a Chihuahua 6" sides were plenty tall enough to keep it contained.
He still prefers land based but nice if he needs late night relief and you don't want to break out the dingy.