You won't be able to cut it straight with a hacksaw. Do you have a router?
Will the edges be covered with anything? THere is often a lip on the edges to help keep water from rolling off.
Strap a straightedge to the bottom of the thing and use your circular saw.
If you have a belt sander, you could use that.
Do a search here...someone did this a few months ago (search is in the blue bar near the top).