I have used JB Weld to repair a line gouge on a Farlows Grenaby, and used it to repair a cracked plastic bushing on a Shakespeare 2052DA spinning reel with good results. On the Grenaby, I applied a small layer of JB weld, let it cure, and then repeated the process until I built the JB Weld up to the appropriate level. I smoothed the repair with fine sandpaper.
Since the repair on your Speedex is quite a bit larger than a line gouge, I would recommend you construct a rudimentary mold using thin plastic and tape to hold the JB Weld in place as it cures. After it fully cures, you can final shape and smooth the repair with fine sandpaper.