I realize that this thread is old but the problem remains the same. As I get ready to change the tires on my 2015 R3T, I noticed the big allen head needed to get the axle out. I found this thread and it is very useful. I went out and found that the plug socket tip worked beatifully. I tried the four of five standard plug sockets that I had and they all worked but all but one was slightly loose but not enough to round out the female allen socket in the axle.
But... just for the heck of it, I checked Amazon and they have a
19 mm 1/2" Drive Hex Bit Socket for $8.99 as I write this. Even though the plug socket works fine, I went ahead and ordered it. Of course, you can probably find the same or a similar adapter easily elsewhere including the big-box stores or local hardware stores. It's cheaper than 19mm allen wrench and you can't set the torque with an allen wrench.
That said, the plug socket thing is a really good idea. I'm just a tool freak and use any excuse to buy a new tool.