i actualy did mine the easy way. i made the front, back and side from a 12 foot piec of 1x12 lumber.
i know how i did it, but explaining it is not my expertice. ill try my best. first off i want to clarify that in the basic construction of the box the two front and back pieces sandwich the 2 side pieces
. i hope you understand that. i measured the side of the tank and i added 1/2 to 3/4". in your case i would have cut the sides 13.00". you dont really need to add much to the sides because the front of the side pieces will end up flush with you tank trim and you dont want the back of the canopy to rest too far behind the tank trim.
for the front and back i measured the length of the trim, added the width of the side pieces of wood(remember that 3/4" plywood is really 5/8" i believe) and added 1". for you it should be 30 3/16(trim) + 10/16(side pieces) + 12/16 (extra)= 31 9/16.
i would round that up to 31 3/4.
your measurement and mine are almost the same for the front and back. its only .5" difference, so you might as well go with you first measurement just in case you mess up the edges and have to do some sanding. the sides are what concerned me since with your measurements you would have the canopy hanging 2" over the back of the tank.