Nice job. I bought one of these mats and gave up trying to cut it to fit. It's a heavy SOB and doesn't cut easily. It makes a great anti-fatigue mat for the work bench.
I did the same. Use a jigsaw w/an aggressive wood blade, and it cuts like butter. I did this with a paper template, then used a router with a 3/4" plunger bit while it was in the bed around the edges, to give me the initial gutter around the edge. I then used a straight edge to go from the gutter, out to the rear. This helps to direct the debris and liquids to the rear, with some relief for the "D-rings" on the floor in the rear for it to fit flat.
I found the thinner mats didn't hold up to the chainsaws, machetes, pole saws, logs, coolers and crap we were throwing in the buggy on a daily basis.