Welcome to the Grumble, JivinMe.
A "slip mat" or slip sheet is just a strip or piece of matboard that is used under the mat you are cutting.
It can be 10" x 40" for most mats or 60" for larger ones and will work fine and you can shift it left or right if needed to get a fresh cut.
It's just to support the cutting area and so the mat cutter blade should go thru your matboard and just slightly into the slip sheet but not all the way thru the slip sheet.
It really has nothing to do with the mat moving while cutting, it just supports the cut and catches the blade to get a clean cut.
The slip sheet can be just a scrap of a regular matboard.
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