I didn't have a problem cutting mine, I used my 45 mm rotary cutter. I can also cut with my left hand for the left side or do the forbidden cutting towards myself no no.
But if you're successful with cutting the 3 sides and it's only the left side that's the problem because of the awkward angle, try tracing that side with a pencil and use a small ruler placed on the outside of the line and then cut it with your rotary cutter as normal.