Just a washer. The deck is over an inch thick and the track is bolted every few inches, the load is liniar to the track and does not pull up on the track. If you have the car at the end of the track and sheet hard that is much higher load than the travler tackle so I thought the stock bolting was good but used a washer for overkill.
The cost of the WM items was about $40. and the bolts, shackles and spacers was around $10. My daughters Hobie cat donated the blocks.
$ for $ I am pleased with the performance.