I haven't used these aftermarket undertray ducts, but they look promising, and are about as inexpensive as I can find.
professionalawesome.com
Professional Awesome Splitter Diffusers - Pair - Professional Awesome Racing - Time Attack and Downforce Specialists
Install tips: Ensure adequate room in front of and behind diffusers to ensure airflow isn’t constricted for maximum performance. Designed to be riveted, bolted or bonded to a splitter. Please ensure your splitter is properly attached to the car. These can, and will, create much more downforce...
professionalawesome.com
