This kit should be included with the XBench, or at least part of it should. Used the adjustable hold-down on a recent project. Easy to set and provides decent holding power. Long-term durability a question since the items are plastic. However, if they're used in the manner they were designed for and not abused, they should provide the average-to-better-than-average user plenty of long term durability.
The clamps and blocks in this accessory kit help make the table much more useful and versatile. Bought this for my husband to go with the workbench. He plans to use the blocks and clamps for woodworking.