I used the rubber plugs hcb3200 shows on my 57, but just for light stuff. I would not rely on them with any level of comfort to secure anything heavy, without attaching them to the bed; which you are trying to avoid.

I went with wooden pocket stakes on my 41; some nice looking hardwood, like oak or something. You can remove them and your bed would remain unaltered.

~ Victor
1941 3/4-Ton Pickup (in process). Read about it in the DITY Gallery
1955 Grumman Kurbside "Doughboy" 235/3 on tree w/ OD
1957 3100 - moved on
1959 C4500 Short Bus "Magic Bus" - moved on
1959 G3800 1 Ton Dually "Chief" - moved on
1958 C4400 Viking "Thor" ~ moved on to fellow Bolter