Barry- Do you have enough room to roll a bit of the paper on the darkroom floor? If so, my technique may work. I cut a piece of cardboard the size I want to trim the paper to. I place that over the paper, just to get the right size. I slide a metal straightedge below the cardboard, lining it up with the cardboard edge. Underneath, I have oversized cardboard, so I can use the straightedge and an Exacto blade.
If you don't have the floorspace, can you borrow a darkroom to cut it in? You'd be welcome to cut it in my DR- I'm in Alexandria.

BTW- Did you get it from the Craigslist posting for "Fumatone"?