The cleanest way is to use a router with a bottom bearing flush trim bit. Obviously you first need to buy an original plexi marquee that is larger than the marquee area of the cabinet you're trying to convert. You clamp the marquee between two solid pieces of wood. You leave the edge you want to cut off stick out beyond the edges of the two pieces of wood (which should be parallel). You then set a fence across the top piece of wood and Clamp it down. Make sure you clamp from the top of your fence to the bottom of the bottom piece of wood. That holds everything together in a big sandwich. Then take your router, set the base on the top piece of wood, the edge of it against the fence, and zip off the offending Marquee edge. You do this on two sides or four sides or whatever you need.