I have a knockbox just like this one. The rubber coating on the bar is starting to fall apart. Any suggestions on a replacement (not to the knockbox, just the rubber part). I see WLL will sell the entire bar but the metal part on mine is just fine, I just need to replace the rubber cover.

You could try buying some tubing of the right size and thickness at a hardware store. That's probably what I'll do if mine ever needs replacing. I have the same knockbox, found it at a thrift shop.

Reading John's reply, I'd probably go with his recommendation.

A piece of reinforced fuel injection fuel line would probably last indefinitely. Available at any auto parts store.

I used some water tubing i got at home depot on a knockbox that was very similar to yours. Hasn't worn out since (years).
The rubber thingies at the end are replaceable by grommets used as pass through in electrical boxes...those keep getting cut through on the sheet metal though.