Those in the link are hoffman knockout covers designed to plug a knockout hole in thin metal like a breaker panel. The motor has a thicker aluminum casting where the plug fits, it would be a difficult place to install one and will require removing the end cap. These plugs have a wing nut and I'm not sure it would fit in the space. While it might work, a metal threaded plug would be much easier. These type plugs usually have a square, or hex (allen wrench) head, half inch is the size you need. One of these will tighten and stay much better than the plastic junk they put in there from the factory.
Incidentally I also have this same pump, about 2 months old. I was just about to go shut my pump off anyway so I took a look at it. My plug appeared to be deep down inside the hole. Much further than the side plug. I came back in and got a screwdriver. As soon as I touched it, the plug fell inside the motor!
It could catch and tear up the armature, or it looks like the motor may have brushes too, so the commutator and brush holders could be damaged by this thing floating around in there. I took the plastic back cover off and was able to use a zip tie to push it, the plug fell all the way to the bottom and fished it out. I pinched the plastic plug threads with some pliers to damage the threads a bit, then threaded it back in and it tightened up as it got flush. It would probably stay now, but I'm going to find a couple metal plugs tomorrow and fix it right. With a little thread sealer it will probably outlast the motor.