Oh right sorry, the 275 I have for sale is the ds version with the sliding table, I thought yours was the same. Norm Abraham did a good video on making a sliding sled for a table saw, it's on YouTube if you have a search
Different saw, but as has been said, decent quality birch plywood for the base. Don't go too thick otherwise your depth of cut will be reduced too much. I went for 12mm Baltic birch.
For the runners you probably should use something stable, that doesn't move with moisture/heat. I bought the squarest flattest plastic chopping board I could find (Tesco value) and milled strips from it to precise thickness of the mitre slots.