After our first attempt at a home built proof of concept solar thermal panel, we were a bit disappointed with the results. It took about 4 hours before the thing started work, and was a bit costly (at over 50 dollars) to make.
I knew it could be done better and cheaper. My first mistake with the first one was purchasing everything new. With ample reusable resources at the local town dump I knew it could be done on the cheap.
Another flaw from the first panel was using pond liner as our collection medium. Pond liner is plastic, does not absorb heat as well as other materials (like metal) and is harder to work with as you have to use glue or tape to create an air pocket to hold the water. It leaked the first couple of times we used it and took extensive repairs to make it work.
We solved this problem by using a ready made collector. Something that was already designed to distribute heat, and made of metal.
The last major flaw in our first panel was using plexiglas for the cover. Its hard to work with as it will crack, and using two pieces left a hard to close crack in the middle.[source-http://www.thesietch.org/projects/solarthermalpanel2/index.htm ]