The factory top case is about the same size as the Emgo Travel Trunk, and the Emgo Travel Trunk costs $80. I have the Emgo on my Weesys and it wasn't much trouble to attach, though I did have to drill or modify holes in the Emgo mounting plate to be able to mount it onto the Weesys luggage rack (and had to use generic bolts, not the ones that came with the rack). The Emgo fits my size Large HJC full-face helmet perfectly.
2018 Versys-X 300 ABS. Former bikes: 1983 Honda FT500 Ascot, 1985 Honda XL-350, 2001 Kawasaki KLR-650, 2003 Kawasaki Concours, 2008 Suzuki V-Strom 650.