Beyond Ambroid
A quick patch that I have used for years is plastic electrical tape. It adheres well to fiberglass and canvas. I have used it to repair holes and gashes in canvas. One temporary patch was on the fiberglass canoe for two years and was still going strong. I used to tape part of a roll on the center thwart,it weighs nothing and it will always be with the canoe. It is now available in colors other than black at Home Depot. For that L-shaped patch I would first sew the canvas shut using the baseball stitch and curved needles. The stitch pulls the edges together flat than cover with electrical tape. One such patch on a friends canoe lasted 4 years. And was going strong with a new application of new tape.
Bill