Is there any way you can put your cutting mat on a table or counter? Your knees and back are going to hurt big time if you are on the floor. This, in turn, may cause you to have an accident with the cutter if your are tired/hurting.
The best cutter I find is the Olfa cutter that closes automatically when you release it. Mine has a lock that can be put on when it is opened or when closed. I usually do not lock when it is opened as I what it to close whenever I release my hand. I was using an older cutter the other day that stays open until you close it and it made me nervous that way. It is so easy to become inattentive after you get used to the cutter.