I have a HF Lewis and I got some black rubber tubing/hose (not gardon hose, this is like lightweight shiny black tubing about 1/2" wide) from the lawn irrigation section of the hardware store. I screwed one end of the tube into the Lewis (the underside so it's less conspicuous) then I wrapped it several times and screwed the other end of the tubing to the gun the same way. Make sense?
On the sling, as much as I'd rather use leather, I believe a khaki colored web sling is accurate (like a Boy Scout belt material).