Purchased the A3 clear book for $2 Cdn only from Daiso Japan in Richmond, BC, Canada.
This is the simplest DIY project I've done so far for Onewheel, and the result is satisfying. Curved the sides for a slim profile (similar to Float Fender), but the best part is using a 3-hole punch to open the screw holes. The 2 holes at each end of the 3-hole punch line up with the OW fender screw holes. Well, off by maybe 1mm but workable.
Still waiting for the rubber paint to dry when I took the photo. [Edit: the A3 clear book is very easy to cut using standard scissors.)