I recently published a paper that included some research about optimising transparency, I found that, contrary to the conventional wisdom, thicker layers is not the key to achieving transparency. Instead, it is avoiding under-extrusion meaning that slow printing and thin layers can give you good transparency.
The text in this image shows the print characteristics and is read through the device. I used Crystal Clear PLA from Fabberdashery.
Link to the paper if you want more info: http://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0152023