I have a large cake that is going on a 25 x 35 inch board of 1/2" ply wood.

What would be the best way to cover it. Tape parchment paper to it? Freezer paper? Ithink I'm going to put fonant over that too.

Any sugestions.

You can use Contact paper which is a self adhesive shelf liner. It comes in a variety of colours and there's quite a bit on the roll, enough for several cake boards.

Will that stick to the wood well enough?

Absolutely!