It's possible to make a bottom panel without using clamps. I like to use gravity, scrap wood and bricks. The reason it works for me is milled balsa sheets and paulownia strips of matching thickness.
The only drawback is you can only lay up a strip one at a time. It's not a problem for me since I'm not trying to build a bunch of surfboards. I want to give a shout out to my balsa supplier. They have given me good service for the past 2 years. Reasonable prices also.
Yesterday, we had fun waves. The conditions were butter. The water is warming up nicely.