I went to the local reclaimed building materials yard looking for something to use for spacers under my '52 's bed.
I saw some bumpers that they install on loading docks that stop the trucks from hitting the wall.
These were about two feet long by 6" by 4". each section was made from a 4 x 6 inch piece of rubber that was about 3/8 of an inch thick (reinforced with canvas )These were bolted together to hold them stacked. Of course each piece had 2 holes in them.
So I stacked the right number under each of the 6 places needed.
Worked perfectly.Cost me 5 dollars.