Patching the plastic bed rails

Previous owner drilled into the bed rails to mount a tool box. I don't mind the holes in the metal rails but want to patch up the plastic covers on top. Any good products out there for this? Sorry pic is upside down for some reason.

 

Not sure how invisible you could make the repair, but there's a material sold for repairing plastic dock floats. It's sort of like black glue sticks:

http://americanmuscledocks.com/products/float-drum-repair-sticks

I've bought stuff from them; good service.

You might also just be able to plug the hole with a 'christmas tree' fastener:

https://www.belmetric.com/body-shop-clips-common-christmas-tree-retainers-c-12_178.html

These guys are great too, but your local hardware store probably has a selection of these in the nuts and bolts aisle.