Just in case you have forgotten: Bernie is traveling from Canada to Mexico via Mule Wagon. His Mule "Polly" is Barefoot and uses Easyboots during her travels.
Bernie has posted on his website, www.riverearth.com, some tips on how he keeps Easyboots on Polly's feet. Below is a brief excerpt from his website including his own pictures.
Mules have narrower hooves than horses. You could describe a horse's hoof as "C" shaped whereas a mule's hoof is "U" shaped. That means the rubber hoof boots, because they're designed for horses, tend to fall off a mule's feet. To adapt the Easyboot (my brand of choice), I made shims that screwed into the inside of the boot.
Step 1) Round up a 2-degree rubber wedge pad (found at the local farrier's supply). Lay it next to 
the boot's flat side and trace out the boot shape. Make sure the fat part of the wedge pad is at the top. This makes the shim grip better, like the tapered collar on a drill chuck.
Step 2)Cut out the wedge. Drill holes to match the existing boot hardware.
Step 3)Using the screws and hardware found on your EasyBoot, screw the shims to the inside of the boot. You can use the existing hardware, though you'll have to use longer machine screws. You can purchase them at your local hardware store. Remember to use stainless steel.
That's all there is to it.
You can read this and much more on his website.
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