Sod
I got some sod from my yard, and put it in a suitable plastic planter. I placed the planter on my bow, thinking we would use it on the weekends. I thought that by vacation time, dog would fully trained and ready to go. WRONG. Birds (I think) tore it to pieces the moment we left the boat, and there was dirt all over my bow. What a mess. Failure.
Next, I went to Home Depot for sod (no way was my wife going to let me cut another chunk out of the lawn). Home Depot was not happy with the sod, and told me to take what I wanted free of charge. I took home about 3' and put it on a piece of plastic in the yard. In the coming days, after running from it and rolling on it, dog actually went #1 on it. When vacation time came, I made a quick cardboard tray from used boxes for the sod, and placed it on my portuguese bridge (out of range of birds). I covered it until we departed.
Once underway, dog hesitated somewhat but used it for #1. After she used it for #2, it was days before she would use it again for anything. Dog is about 137 years old and we are her second home (rescue).
The sod and box worked out very well. We had a couple of long passage days, and a couple of rough anchorages where I did not want to put the dinghy down in the dark. The sod was magic for us, and it was returned to earth about an hour before making home port at the end of our trip. I will do this again next summer.
Good Luck