I had the same problem and this is what I did. I used 1/2" pvc to make an arch support. I used 2 pieces about 1 foot long each glued together with a tee in the middle. I made two sets like this, these are the "feet" for the arch. They sit on the floor against the side wall of the boat so the arch doesn't fall over. I then took a long piece (you will have to play with the length to get the height you want) and inserted it into each of the tee's to form an arch. For storage, pull the long one from the two feet and it all lays flat. Make two or three supports as needed.
Hope the makes sense and helps