Take it to a custom framing store and ask if they'll trim it for you.

Someone felt like this should be down voted, but I did framing for years and trimming foam board for people was something we did frequently. OP, if you aren't in the mood to play with wire and a battery, this is your next best option for a clean trim.

Although, the fact you've already cut it down means there might not be space to make fresh cuts.