Guys, you can edit your original post to consolidate links. It'll make things easier on @Erniepower
And more stopper bolts with a bumper
https://www.alibaba.com/product-detail/equipment-rubber-stopper-with-screw_60342681792.html
Ernie, why not just dip it? I bet the truck bed liner materials would work like this one.......... https://www.harborfreight.com/124-fl-oz-iron-armor-black-truck-bed-coating-60778.html , or you could use something like this, which is probably more rubbery .... https://www.amazon.com/Plasti-Dip-Multi-Purpose-Rubber-Coating/dp/B00BZBN2DK. Pete
To be honest I really haven't had a chance to review all the answers yet. I have been swamped all day yesterday.Alright so who is the winner already?