Lots of us have a clip-on extension like this. Have a look through the photo gallery.

They're available on eBay, Amazon and a well-known Chinese site, in a couple of sizes and in clear or tinted.
Mine's the large version and with shipping from China, cost me the princely sum of £13.82. It's fitted to a Givi Airflow
adjustable screen for even more height (mine came with the bike)