Drill out the ends of the crack to stop it from spreading, then use that roofing tape. Make sure your boat is nice and dry.
Dave Frank gave me some a few weeks ago. The stuff is amazing. I'd say it is mandatory for the river patch kit. I paddled with a 14" crack in the bottom of my boat for 2 weeks. (the boat is done now, but it kept me afloat for a few laps down SSV, Bear, and Escalante).
If you find some, let me know. I need another piece to patch my other boat. F#$%ing boats.