Welcome from Ontario, Canada. You can use a piece of clear plastic to mark the blocks and then use a regular rotary ruler and rotary cutter to trim the block. You can also sometimes get a place like Lowes to cut you a plexiglass glass square that you can use with your rotary cutter.
I would not recommend using a plastic square you made yourself for rotary cutting unless it is plexiglass. You will end up shaving some of the plastic away with your rotary cutter and ruin your block or cut yourself. When possible invest in a big rotary cutting square ruler. I use my 15 inch square all the time.