I have not substituted it for a sheet of glass (which should work), but instead emailed 'The Farm' which has a prototype design on its website: http://www.thefarm.org/charities/i4at/surv/sstill.htm
This design is expensive and tough to implement in an Indian slum, so I wanted to know if I could use local materials. i also talked to friends about what we can do. Here is the answer from The Farm:
I agree that silicone and rubber caulking are high-tech items. I am sure there are local substitutes. Lately we have been using an elastomeric paint, but again, that would not be something easy to acquire in the poorer sections of India. Any black paint, if it is not lead-based or otherwise toxic, might work if applied with enough thickness and perhaps sealed with a clear coat of some material that would not easily melt. Perhaps if a layer of fine charcoal were laid across the bottom, no black paint would be needed and the charcoal would also filter and protect the condensate.
Good to know you are experimenting and we would be happy to publish any new designs to our site.
My friend also suggested that we add a reflector to capture as much sunlight as possible, and instead of a plastic blox use aluminum. All this will add to the cost but we need to implement it in India to actually test the results.
My friend is working on a design diagram. I will have it published when it is done!