How can I use sunlight to heat a RBF and reflux a reaction?

This may seem like a off topic question but I really need help on it. I am currently working on a polymerization reaction in which I'm breaking down plastic polymers into smaller molecules. However, since this is a green chemistry project, I decided, with the suggestion of other chemists, that I should a method that doesn't require a lot of electrical energy to heat it. One idea was an enzyme, but I decided I want to use solar energy to reflux the RBF. I have a few ideas and please comment on them and also provide your own suggestions: 


* First is using a parabolic mirror below the flask to focus the heat towards it.
* Second idea is painting the RBF black and hoping that is sufficient to heat it to reflux.


