I had the same problem with mine.
I used a combined method
1/ Fill cracks with "shoe goo".
Then when that had set,
2/ Got some large heat shrink tubing, covered the boots with that, then used the oven on low (Do NOT tell my wife!) to get it too shrink and tightly cover the boot.
It appears to have worked well with mine.
I did not need to do anything on the inside of them, even though a couple of the cracks were right through.
I made sure the ShoeGoo was squeezed right in to the cracks before I let it set.
The heat shrink was about 100mm, with a 2 to 1 shrink ratio, so it ended up as a tight fit on the boot after I had shrunk it.
I could probably have used 90mm, but could not get that in black at the time.