Hey John, I did a lot of research on the same problem last summer and fall. Even visited some canvas shops. Same old story you get what you pay for. It basically came down to one portable tool. The Press and Snap.
Here is the link
http://www.pressnsnap.com/pns.htmNot the cheapest, but it is suppossed to be one of the best. I have not purchsed one yet but I plan to. Has to be better than all the cheap one time use tools out there.