If you don't mind taking a heat gun to the foam, you can make any shape with enough time. It took me about a week of evenings to do the bottoms of each hull. The sides were done in about 30 mins each hull.

One alternative for a simple hull build, I would consider making the hulls with chines using thicker foam with one skin already on the inside then once all the structure is in, torture board (sand) some round into the chines before glassing the outside, if you want a round hull. I like how the paper tiger is made though, leaving the chines in. It looks alright and goes well for a small cat.

If you want a really easy build, consider a similar method to stitch and glue for plywood, but with just the foam, or just one skin on the foam, then laminate once the desired shape is achieved.

Or for a well rounded bottom, mould up just the bottom of the hull to the point where the flat sides start. Once this is made (with a single skin flange to attach the sides to), install some well thought out frames (pre-fabricated frames with flanges would be awesome), glue the pre-laminated flat sides on, then glue on the deck. This may be a good alternative for any cat build actually, anything that has relatively flat sides. You've got me thinking now, damnit, I was just about to get some sleep!