A block of wood - nothing special, I'm using an offcut from an ikea hack. It's 1 1/2" deep. You could easily use a 2x4 off cut. You may just need to make more cuts. A much larger piece of wood will necessitate a longer bolt, too.
2 large (1" or larger) washers and 2 small (1/2" or so) washers. If you have large fender washers, that will work,too. These act as bushings, so don't skip there