The covers just snap on. If the foam doesn't last, I can replace it fast. Heck, it cost two dollars!

By the way, this tool from SailRite is a real value for the money. They're just little snap fasteners that slip over a pair of Vice-Grips, and cost
1/4 the price of a true snap fastener tool.
If you only do snaps occasionally (like me) there's no point spending $200 on a real snap tool, and this one works great and will last forever -- if you don't lose the little things.

This is just foam pipe insulation
from Home Depot. It's 1 1/8" inside diameter, so there's room around the stainless to let air in so it can dry out.

The covers are made from Sunbrella scraps.

They're about an inch longer than the foam, and I tacked the edges down as shown so that they'd hug the rail.

This is Tenora (aka GoreTex) thread. It's tough stuff and
isn't affected by UV.