Mercedes W123 300TD Roof Rack Cross Bar Question

[johnbob]

I recently purchased a set of these vintage roof rack crossbars but they don't have the black rubber pieces (see my photo for my example) that go along the bar. I'm assuming they are no longer available anywhere so I was wondering if anyone on here has made some or found a similar substitute? They are basically just rubber pieces, it can't be that difficult to figure something out.