First Time Rink!

First thanks for having me.   I live in S.E. Mass about an hour south of Boston.   My daughter plays hockey so i felt that it would be great to build her a rink of her own so i can skate with her whenever and she could have friends skate with her as well.   Any way i ASSUMED my yard was pretty level and bought a white 40x50 tarp (which seems fairly rugged).   I was given 14 pt 3x8x8's plus I purchased a couple of 2x8x8 and "tacked" it down this past Saturday.    (Planned rink was/still maybe 32x45) My neighbor came over and I ask him how level he thoughs my yard was and he said, "We may have to raise some boards but i don't think we'll have too much of a problem".

He grabs his fancy string and level gadget and after about 15 minutes we can tell the flattest area of my yard (NEVER EVER CONFUSE FLAT FOR LEVEL) has a pitch of 13 inches.. (Grrrrrrrrrr)  With water being what it is i want at least 3 inches + of ice at the most shallow(WEST) end and i guess it will be 16 inches + at the deepest (EAST) end.      The dreams of dropping and pinning my 3x8's are pretty much gone.    

So here are my first round of questions.    

1.  I want to build 20 maybe 24 inch boards for the deepest end.   Whats the thinnest plywood i can use if i sure them up with 2x4 framing in the back?

2.  Does it (plywood)  have to be PT (pressure treated)

3.  With all this water/ice at the WEST end (16 plus inches) how much bracing should the boards have to i don't risk a break?

4.  Can i double up on 8' boards?  (raising them on blocks and screwing another underneath?   for those 7' plus areas and stake the heck out of them  so they hold) or is this next to impossible?

(As you can see I have enclose a pic of day 1 (When I was trying to fool myself this thing would hold water)   I have marked the pic up a bit so I can get some tips.  Thanks again guys.

Todd