The Riser plinths are used for leveling your shower trays and we stock in quadrant shapes and square and rectangular shapes, ideal for when fixing to a solid floor and you need to raise the tray from the floor and indeed allow easy access to your waste by removing the plastic plinth. The Extension panel listed is for when you require a longer size than 1200mm ( up to 1800mm) when you purchase this extension kit we will send the square kit feet and add the extension package automatically , need any more help give the sales team a call to discuss