I've always played it with mostly my thumb, except for the last note of every chord form with my index, as it's chord shapes are all picked sequentially downward from low to high. I can use my other fingers well enough, and in some songs their absolutely necessary, but this isn't one of them. In my experience, trying to use them here makes it harder than it has to be.

So from the top it goes:
P=thumb I=index

....P...P..P...I......P..P...P....I....
E|----------0------------------|
B|-------------------------1---|
G|-------2-------------2-------|
D|----2-------------2----------|
A|-0-------------0-------------|
E|-----------------------------|

And so on for the rest of the chords.