Blimey, seam grip it is then!
Are you selling them, and so putting tape on it for aesthetic purposes? If not, I'd trim one seam allowance enough that you can fold the other over it, and then sew through that.
Edit: just realised you meant waterproofing.
@Kurai started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
Blimey, seam grip it is then!