For all things feet related, try Leukotape.
I taped over 20 people's feet on the AT last year.
Helps preventively as well as for blisters, callouses, and hot spots.
I used it extensively for heat rash if my pack rubbed, on my hips, under my arms, everything.
It it the primary piece in my emergency kit.
That, and an empty chapstick tube of needles and thread.
I kept a copy of the book: Fixing your Feet by Dr. Jon Vonhof on my phone,
for pics of those tough to tape areas. (He is a physician who tends to ultra runners' feet).
LEUKOTAPE is worth the 8 bucks. !!!!