look for a source of cockpit coaming xl

You can use black rubber (or, more likely, synthetic) tubing and cut a slit in one side. The best way I've found to make a straight cut is to drill a hole just a bit larger than the tubing in a block of balsa and then stick a no. 11 blade through the block so the tip reaches a little bit into the hole. Then pull the tubing through the hole. The cut doesn't have to be perfectly straight. There are probably other and maybe better ways, but this will work.

Depends on the scale of the model, how scale you want it to be and the era. As mentioned a rubber tube slit down one side works. Proctor offers several sizes of a shaped tube that is slit. You can find them here: Proctor Cockpit Trim

If you're an anal scale fanatic like some of us here, some thin leather works and is typical of the real thing. Finding it can be a challenge but I've had luck in the past obtaining scraps and leftovers from these people: Fox Creek They also make some fine motorcycle gear if you're in to that.

Try this link.

Fourmost Products Trims & Flanges | Fourmost Products

1/4 scale Flair Stearman part built.
Ebay is a cheap source of leather scraps. I bought a mixed bag for a couple of £. Did my 1/4scale Flair Stearman.